Example: Strings and piano on the MODX, lead synth and effects on the MX61. With all parts of the MODX

Yes. Totally possible.

When you use the MODX as the master keyboard (“master keyboard” defines the set of keys you are physically placing your fingers on), you can have eight Parts 1-8 play. This means the MODX is an eight channel master keyboard controller. The eight Parts are on Channels 1, 2, 3, 4, 5, 6, 7, and 8.

When you use the MX as the master keyboard, you can have it trigger any sound in the MODX, by simply transmitting on the MIDI Channel that matches the Part number you wish to sound.

So say you put a 3-Part MODX Strings, and 4-Part MODX Acoustic Piano to sound together when you physically touched the MODX keys… and at the same time, you could put your lead synth sound, w/effects in Part #8 and trigger it using the MX keys, by transmitting from the MX on MIDI channel 8. Easy-peasy.

You could connect your MX to the MODX via MIDI and control any MODX Part from your MX by transmitting on the appropriate channel — by matching the Transmit Channel of the MX to the Part you want to sound in the MODX. If you transmit on 10, the MODX Part 10 will respond. In this scenario touch the MODX keys will play as many as 8 Parts 1-8, while the MX triggers any single Part.

If you want to use the MX as the Master Keyboard (play physically on its keyboard) MX —> MODX, if you don’t want sound from the MX you would set Local Control = OFF. This would turn the MX into a ‘silent’ master keyboard. If you transmit Out from it to the MODX, you can trigger all the KBD CTRL linked Parts of the MODX together — it can simply take the place of the MODX keys. You do this by placing the MODX in MIDI I/O Mode = Hybrid or Single

Or if you set the MX to transmit on Ch9, you can have the Part 9 sound from the MODX Tone Generator… while triggering MX keys

In MIDI Spec 1.0, the keys you physically engage are referred to as the “Master Keyboard”… and the receiving device is considered the “Slave” Tone Generator. (I suspect in our politically correct world, those terms to soon be dropped…) but the significant thing to know is the MODX sounds come out of the Main L/R Outputs of the MODX, and the MX sounds come out of the Main L/R Outputs of the MX… no matter what MIDI is doing or in which direction you go.

THIS MEANS: you still must connect both to a sound system if you want to hear them both.

Here’s what you need to know about the MODX and MX:
They both are capable of 16 Part multi-timbral setups, where each Part has a different instrument program. (Great for MIDI Sequencing)

In the MODX, you can link as many as eight of Parts together, 1-8, for simultaneous play using the green KBD CTRL icon on the HOME screen.
In the MX, you can link as many as two Parts together, 1-2, for simultaneously play using the [LAYER] or [SPLIT] buttons.

The MODX has a built in 16 Track Performance Recorder (Sequencer)—that features a linear MIDI Song mode and a looping Pattern mode.

While you are playing the MODX keys, you can have the Sequencer trigger the MX via MIDI.
As you get to know your MODX, you will start to realize that very rarely are you layering 8 MODX sounds to play simultaneously. At first, that is what everybody thinks they are going to do, but hopefully, you outgrow that… just because you can layer and split as many as eight sounds simultaneously, you don’t have to… you wind up doing a lot more with less.

When using the MODX for live performing, and using it as the master keyboard, you could create an eight Part Performance where 7 of the Parts come from the MODX and the 8th Part is triggered from the MX via MIDI.
You could create an eight Part Performance where 6 of the Parts come from the MODX, and the 7th and 8th Parts are triggered from the MX, via MIDI
You could create an eight Part Performance, where 5 of the Parts come from the MODX, and the 6th, 7th, and 8th Parts are triggered from the MX via MIDI… And so on. Any combination that adds up 8.

Because the MODX is an eight channel Master Keyboard Controller. This means, that 8 is the MAXIMUM number of Channels you can Control from the Keyboard simultaneously. Those 8 sounds can come from the MODX Tone Generator, or from the MX Tone Generator, or from any combination of both (that adds up to 8).

The Sequencer has 16 channels. Don’t mixup physically playing the Keyboard, with playing back a Sequence.
The Keyboard can make 8 things sound at once… be that from the the MODX or in combination with the MX.

If you MIDI OUT of the MX —> to MIDI IN of the MODX… you can have the MX Transmit to any single sound in any single Part, 9-16
You could also set it so the MX Keyboard could trigger the MODX Tone Generator and play all the KBD CTRL Parts together…

How can I keep playing the first 8 parts on the MODX and the other 8 - 16 on the MX61? In addition, that I can continue to use layers and splits. So all parts come from the MODX

The best solution is to add an additional device, or at least, some other software. But before I get there, here's what you can do "out of the box"...

As supplied by Yamaha, the MX has only the following two MIDI transmission capabilities:

1) It can transmit on any ONE channel. You can actually use the 16 sound select buttons to change this channel on the fly. With MODX MIDI I/O Mode set to Multi, the MIDI channel a given Part receives on is the the same as its Part number. Put all that together, and it means you can press button 9 on the MX to play the MODX Part 9 sound, press button 10 on the MX to play the MODX Part 10 sound, and so on. This is a partial solution to your question, It allows you to "keep playing the first 8 parts on the MODX and the other 8 - 16 on the MX61" BUT the MX61 will only play ONE of those Parts at a time.

2) It can transmit on TWO channels (split or layered), but those channels can only be channels 1 and 2. So in this case, your MX can play TWO of your MODX Parts at a time BUT they will be Parts 1 and 2. (You can turn off KBD CTRL for those Parts on the MODX if you want those two Parts to only be triggered by the MX, leaving the MODX's own keys still free to simultaneously trigger any of the sounds in Parts 3 through 8, split/layered as you please. At least I think this will work. 😉 )

I'll also mention that by changing the MODX mode to Single, it is possible to use the MX to play any combination of sounds in parts 1-8 (they will respond to a single MIDI channel input from the MX), which gives you a way to play more than two MODX parts from your MX, but (a) these will be Parts located in 1-8 rather than 9-16 as you requested, and moreover, (b) the MODX itself will only be able to play one Part that is not being played by the MX (and it will need to be a Part you have located somewhere in the 9-16 range, IIRC).

So there are lots of combinations available, but none are exactly what you asked... 1-8 from the internal keys, 9-16 from the MX keys. For your example of (I think) playing 2 split or layered MODX sounds from the MODX keys and two other split or layered MODX sounds from the MX keys, technically, I believe option 2 above can do that, if you put the two sounds you want to trigger from the MX in Parts 1 and 2 (with KBD CTRL off on those Parts), and put the two sounds you want to trigger from the MODX in Parts 3 and 4 (or any other Parts in the 3-8 range, with KBD CTRL on)... but that's a solution for just the one particular example you happened to mention, and doesn't really cover the range of what you're talking about in the main question. You could not use that approach if you wanted the MX to trigger three MODX sounds instead of two, for example (while still having the MODX' own keys trigger two or more Parts).

In short, with the methods I described above, I believe these are your options for triggering MODX sounds from the two sets of keys:
... one Part triggered from MX's keys, up to 8 Parts triggered from MODX' keys (the MX part would be located in the 9-16 range, at least if you want 1-8 all playable from MODX' keys)
... two Parts triggered from MX's keys, up to 6 Parts triggered from MODX' keys (all parts would be located in the 1-8 range)
... one Part triggered from MODX' keys, up to 8 Parts triggered from MX's keys (the MODX part would be located in the 9-16 range)

Okay, so now, how do you REALLY do what you want? Two possibilities:

(1) You add a device that will re-route one MIDI channel output from you MX to channels 9-16 of the MODX. Now you'll be able to play any combination of up to 8 MODX sounds (Parts 1-8) from the MODX' own keys, and any combination of up to 8 other MODX sounds (Parts 9-16) from your MX. You can use a standalone box... the MIDI Solutions Event Processor Plus should be able to do it. Or you can use any of numerous apps on an iOS device (iPad, iPhone, iPod Touch) or Mac or PC. Some iPad apps to look at, for example, could be MIDIflow, Camelot Pro, Keystage.

(2) With additional editing software (the MX tools from John Melas, or the Vycro tools which seem to be discontinued but you may be able to locate online), you can add a lot of capabilities to the MX, storing programs that contain MX configurations that are impossible to create from the fornt panel. I think one or both of these could allow you to rechannelize the MIDI Output of the MX.

BadMinster and AnotherScott thank you very much for your very enlightening answers. I can do a lot with this. I now understand the possibilities and have had tips about additional software.

Thanks again and greetings from the Netherlands 🙂

Camelot Pro has an iOS, MacOS, or Windows choice of host operating system. There's a video showing how to target all 16 channels of MODX using an MX as the master keyboard outputting only a single channel and then re-channeled through Camelot Pro so you can extend the MIDI capabilities of MODX.

Here's the link to that video: https://youtu.be/5Aq0cN5cDok?t=536

... it just helps visualize what's being referenced here regarding MIDIflow, Camelot Pro, and Keystage.

In the video he is dealing with the Montage during the demo. The MODX operates in the exact same way - so everything discussed applies equally to MODX. This allows for using a 1 channel external MIDI controller (like the MX) to control MODX while MODX is in multi-channel MIDI mode.