Yes. Just make sure your external keyboard has a 5-pin MIDI jack (not just USB). You will be able to trigger any single-part Performance, and there is a single-part Performance that has layered piano and strings.

... or you can stick a DAW or other hardware between and route the channels to play strings/piano on different channels.

Unfortunately, single-channel mode doesn't work for this because you cannot tell a PART to ignore external MIDI - or set the note range exclusively for responding to external MIDI.

... or you can stick a DAW or other hardware between and route the channels to play strings/piano on different channels.
Unfortunately, single-channel mode doesn't work for this because you cannot tell a PART to ignore external MIDI - or set the note range exclusively for responding to external MIDI.

As I understand, it's not possible to trigger layers or splits on the same MIDI channel.

I would like avoid to use computer in a band, so is there other hardware that can route MIDI channels?

Not sure if an iOS device counts as "a computer" or not. iOS and Camelot can handle this.

Dedicated hardware:

http://www.ees-musik.de/html_gb/produkte_procon_gb.html
This device "multiplies" a single channel to up to 16 other channels:
http://www.ees-musik.de/html_gb/produkte_mulkan_gb.html

MIDI Solutions Event Processor Plus

... there are other options.

BTW: you can do layers and splits on a single channel. What you cannot do is differentiate between local keys and external keys because single-channel takes away one of the tools we had to accomplish this (keyboard control). Keyboard Control works differently in single-channel mode. In multi-channel mode - keyboard control=off would allow for you to have a PART ignore local keys but still accept external MIDI. In single-channel mode, keyboard-control=off will ignore both external MIDI and local keyboard keys. So it's not so useful. There are other details. The consequence is that you cannot select PART(s) which will filter external MIDI and still respond to local keys.

So the problem isn't assigning splits and such. It's how to handle PART(s) you want controlled by local Montage keys in single-channel mode (and ignore external MIDI for those PARTs). Otherwise - if you, as a thought experiment, remove the requirement to play local Montage keys - you can have splits and layers all over the place working fine and controlled by an external MIDI master controller.

As I understand, it's not possible to trigger layers or splits on the same MIDI channel.

You can definitely trigger layers, splits and/or split/layer on a single MIDI Channel via MIDI. You can play the most complex of the MONTAGE/MODX Multi-Part Performances from your one channel controller... and you can be simultaneously playing a Single Part that has been “merged” to that same Performance using the local MONTAGE/MODX keyboard.

Here’s what you can actually do... and how it would be done.

If your MIDI Controller only Transmits on a single MIDI Channel...
You can address one Multi Part Performance and one Single Part Performance... such that if you’re triggering the MULTI PART via MIDI, then you are playing the Single Part with the MONTAGE/MODX Keyboard. Conversely, if you are playing the MULTI PART program using the MONTAGE/MODX Keyboard, then your External Controller can play the Single Part selection.
(It is your External Controller that has the limit of one Transmit Channel, if you use the MONTAGE/MODX to compensate for its lack of multiple transmit channels, then you are limited to communicating with just one slot from the MONTAGE/MODX keybed).

This means any Parts you want to play simultaneously from a single set of keys must be linked by the KBD CTRL (Keyboard Control) Function. Any instrument sound you want to additionally communicate with will be *selected* individually by communicating with its slot, directly.

As many as 8 Parts can be linked under KBD CTRL and then pick any one other Program slot to address as the Single.
One Multi + One Single.
This would be accomplished using the MIDI I/O Mode setting.

[UTILTY] > “Settings” > “Advanced” > “MIDI I/O Mode”

Example, you recall “Seattle Sections” (7-Part layered/split String Orchestra) and you “merge” the Single Part lead “Wind Synth” to Part 8.
The KBD CTRL (Keyboard Control) icon is lit green (active) on the Parts you want to play linked as a Multi, in this case Parts 1-7.
The “Wind Synth” is in Part 8, no KBD CTRL icon lit (inactive).

If you want to play the “Seattle Sections” 7-Parts from your External Controller pick a MIDI Channel you want to use and set the MONTAGE/MODX’s “MIDI I/O Mode = Single” and set it to that same Channel. Any channel can be used 1-16.
Your external keyboard will play the “Seattle Sections”, respecting the Note Limit ranges and velocity ranges as set in the program.
(Of course, via MIDI the MONTAGE/MODX respects the Note Limits as set in the Program... not sure where that comes from... if you want to create a split between what the external keyboard is controlling in the MONTAGE/MODX just set the Note Limits as you desire).

You can play the synth lead, “Wind Synth” on the MONTAGE/MODX Keyboard by simply *selecting* Part 8, directly, and playing on the local keys. As long as you directly select the Single Part, your local MONTAGE/MODX Keys will trigger just it.

The “MIDI I/O Mode = Single” definitely does allow you to define which sounds (slots) respond to incoming MIDI (it is just incorrect to say otherwise). It is accomplished by defining which Parts will respond using the KBD CTRL icon... it literally translates to those Parts that will be under the Keyboard’s Control. If that Keyboard happens to be External via MIDI is not an issue. The KBD CTRL icon links the Parts as a Multi... Those Parts not linked do not respond to incoming MIDI...

To flip-flop this (that is, say you want to play the big strings with the MONTAGE/MODX keys and have the external controller just play the lead), set the MIDI I/O Mode = Multi... play the MONTAGE/MODX keys to trigger the KBD CTRL Parts 1-7, and set your external controller to transmit on Channel 8, to trigger the “Wind Synth”.

When “MIDI I/O Mode = Multi”, the synth lead in Part 8, is on MIDI Channel 8..

Summary:
One Multi Part + one Single Part.
The Multi Part can have as many as 8 Parts linked by KBD CTRL
The Single Part is always addressed individually.
What you can’t do is play Multi Part programs from both, simultaneously, this is only because your external controller only Transmits on one channel.

Hope that helps...

As I understand, it's not possible to trigger layers or splits on the same MIDI channel {from an external keyboard}.

One way to do it was illustrated in my very first response in this thread... select (or create) a Performance that has layers or splits within a single Part. It works because the splits/layers are done at the element level... this has limitations compared to splitting/layering multiple Parts, but can still be a useful approach.

A second way is illustrated by Bad Mister's response above this one. It allows you to have up to 8 split/layered Parts triggered from your external keyboard. It works by not permitting splits/layers from the board's internal keys. To restate his summary, with a single channel external controller, you can always create combinations of (a) splits/layered Parts on the internal keys with a single Part triggered by external keys, or (b) splits/layered Parts on the external keys with a single Part triggered by internal keys. What you can't do is splits/layered Parts on the internal keys with different splits/layered Parts on the external keys. (Though again, you can even get around that, somewhat, by creating single Parts that, themselves, contain splits or layers... and there happens to already by one with the piano/strings layer you specifically asked for.)

A third way is to use external hardware. Which brings me to:

I would like avoid to use computer in a band, so is there other hardware that can route MIDI channels?

Yes. Here are two ways to go:

First approach: dedicated hardware. As Jason said, the MIDI Solutions Event Processor is one piece that would work. An easy way to use it is to set it to take MIDI on whatever channel your external controller transmits on (probably 1) and redirect it simultaneously to MIDI channels 9 through 16. Now your internal keys will always play whatever Parts you've assigned them in Parts/Channels 1 through 8 while your external keys can play whatever parts you activate for them in Parts/Channels 9 through 16. The drawback is that you lose Seamless Sound Switching between Performances. If you want to maintain SSS, instead of setting up your device (and your Performances) reserving 1-8 for internal use and 9-16 external use, you can instead dedicate whatever combination you want to each keyboard out of just the first 8 Parts on a Montage, or the first 4 Parts on a MODX. For example, on a Montage, you could decide to universally assign Parts 1-4 to internal keys and 5-8 to external keys (permitting up to 4-Part splits/layers on each of your two keyboards), or you could decide to universally assign 1-5 to internal keys and 6-8 to external keys (permitting up to 5-part splits/layers on the internal keys and up to 3-part on the external), or any other combination that doesn't exceed 8 Parts total. MODX is much more limited here... the only combination that supports splits/layers on both the internal and external keyboards simultaneously (and still maintain SSS) would be to split them 2 and 2, e.g. reserve Parts/Channels 1 and 2 for internally triggered sounds, and Parts/Channels 3 and 4 for externally triggered sounds. This is because of a limitation discussed in another thread here. While it is well known that MODX SSS only works on Performances that have up to four parts, something that had not been clear is that they must be the first four parts, which unfortunately makes this approach more restrictive (i.e. preventing you from easily switching among "3 and 1" and "2 and 2" combinations without losing SSS, as you could if SSS worked with Performances of up to any four Parts, as opposed to only the first four Parts).

Second approach: an iPhone, iPad, or iPod Touch. As Jason said, it's a question as to whether you consider these things "computers." But besides the Camelot Pro software Jason mentioned, I believe Keystage, MIDI Breakout Box (cheapest), MIDIflow, and (for only up to 4 Parts) iMidiPatchbay will all work. Some will work on any iOS device, some are iPad only.

Thank you guys, very detailed responses!
As I already mentioned, I do not yet have a Montage or MODX, so for me it looks a bit complicated since I have to learn about Yamaha Performances, Parts, Live Set etc.

My current setup are Roland MKB A-88 (lower kbd), DSI RevII (upper kbd) MacBookPro with Gigperformer and a bunch of vsti's, iPad, usb docks....

I would like to reduce to a minimum: Montage or MODX and A-88, only two keyboards
So it seems possible to play splits and layers on both, Montages keys and external keys (A-88) simultaneously, all sounds coming from Montage or MODX, right?
There is a restriction with MODX Seamless Sound Switching..., what when I don't need SSS?

As I already mentioned, I do not yet have a Montage or MODX, so for me it looks a bit complicated since I have to learn about Yamaha Performances, Parts, Live Set etc.

A Part is a single sound. It can be a bit more complicated, as there are some single sounds that can consist of more than one part, and some single parts that can include more than one sound, but let's ignore that for now, to help get you oriented.
A Performance is a combination of up to 16 Parts. They can be triggered either internally or externally in various combinations, and in this thread we have been discussing some permutations of that.
A Live Set is a screen full of buttons that call up Performances that you have arranged for convenient access.

There's more to it, but that gets you the gist, for most typical purposes.

So it seems possible to play splits and layers on both, Montages keys and external keys (A-88) simultaneously, all sounds coming from Montage or MODX, right?

Kinda. If the A88 only transmits on one channel and you don't want to add any dedicated hardware or a computer or an iOS device, then there are the limitations discussed, meaning that to do splits/layers on both boards at the same time, one of the boards will be limited to a split/layer that is generated within a single Part, which gets into stuff that is going to be complicated for a new user, and limited in its functionality. It happened to be the perfect answer for your question as originally posed, because Yamaha happens to supply a single Part that actually includes piano and strings. But in the broader scheme of being able to combine sounds however you want, it's not really the answer for most people. Again, there is no problem playing a split/layer on one of the two boards and a single sound on the other (doesn't matter which is which). But if you want to play (different) splits/layers on both boards simultaneously, and your A88 only sends on one channel, then you really are going to want to add something like the Event Processor or an iOS device to the rig to get it to do everything you want.

There is a restriction with MODX Seamless Sound Switching..., what when I don't need SSS?

I'm not sure I understand the question. SSS works as long as you don't use parts 5-16 in the Performances you're switching to or from. So maybe it will help if I sum up the limitations in this regard, relative to the hardware solutions I suggested:

Event Processor set for channels 9-16: You can have up to 8 sounds triggered by your MODX and 8 sounds triggered externally from your A88. You will not have SSS.
Event Processor set for channels 3-4: You can have up to 2 sounds triggered by your MODX and 2 sounds triggered externally from your A88. You will have SSS.
A MIDI routing program on your iOS device: You can have up to 8 sounds triggered by your MODX and 8 sounds triggered externally from your A88. You will have SSS when you do not exceed 4 sounds total (and make them the first four parts of the Performance). If you exceed 4 Parts total, you will not have SSS. (Bonus: If you have an iOS device in the rig, you can trigger sounds from iOS apps as well.)

Thanks for lighten up my Yamaha darkness!
I appreciate

p.s. -- the A88 appears to have a split/layer function that allows it to transmit on two MIDI channels at once. So if you are content to use your A88 to split/layer no more than two Yamaha sounds, you should not need to bother with the Event Processor or iPad app.

Yes, the A-88 has split/layer function, unfortunately the settings cannot be stored.
In live situation it is necessary to change very quickly varying settings.
I have to go with event processor.

Yes, the A-88 has split/layer function, unfortunately the settings cannot be stored.
In live situation it is necessary to change very quickly varying settings.
I have to go with event processor.

If you only need the A-88 to play up to two sounds at a time, it will do the same thing as an Event Processor. i.e. the scenario I described above, "Event Processor set for channels 3-4:". You could similarly set the A-88 to layer channels 3 and 4 and leave it that way for the whole gig. Exactly which MODX sounds it is playing at any given time (and what key range each one is playing over) will be determined on the Yamaha side, based on which Performance you invoke.

The Event Processor uses the same idea... you set it just once, and all further manipulation/assignment is done on the Yamaha side. An iOS device lets you use either approach... you can do it the same way, or you can have the iOS device completely manage the assignments on a song-by-song basis.

If going with MIDI Solutions, I would go with the Event Processor PLUS. More setting "profiles" and more variables can come in handy down the line.