You might want to start by reading this reply by Bad Mister.
https://www.yamahasynth.com/forum/does-modx-support-connecting-multiple-midi-devices-to-it#reply-120111
He explains the difference between multi single and hybrid.

It would also help if you described just what it is you are wanting to do.

Are you wanting to entirely control the Montage from a PC? from another keyboard?

Are you wanting to play some parts of a performance on the montage while other parts are played/controlled by a pc or another keyboard?

Are you wanting to also have the Montage send MIDI to an external device?

The answer to your question, and the selection of Single, Multi or Hybrid, depend on just what you are wanting to do.

As BM explains in that thread the mode setting determines which parts receive program change messages (will you be sending any to the Montage?) and which parts are controlled together by an external device.

The mode setting also determiness which parts send MIDI from the Montage to an external device.

1 When we connect another keyboard from the midi in to the montage, can you explain the differences between the multi single and hybrid options from the utulity midi settings that we need to set?

Multi
If you are set to “MIDI I/O Mode” = Multi the MONTAGE will receive data from your external MIDI device on the MIDI it is transmitting on.
_This means if your external keyboard is transmitting Out on Channel 1, then the MONTAGE will respond on MIDI Ch.1, only.

This is a problem, because…
Say you recall “CFX Concert” - a 4-Part acoustic piano (9-way velocity switching in the main body of the piano sound)… because you are only transmitting on Channel 1, only the first Part (Part 1) will respond. This means you will only be accessing the first 6 velocity layers… the double forte ff, triple forte fff and quadruple forte ffff layers will be missing from the body of the piano sound.

If your external keyboard is transmitting Out on Channel 2, then the MONTAGE will respond on MIDI Ch.2, only. This means, again, you will only hear a portion of the piano, the ff, fff, ffff layers in our “CFX Concert” example.

You cannot trigger all 4-Parts while MIDI I/O Mode = Multi unless your external keyboard can transmit simultaneously on Channels 1, 2, 3, and 4.

Single
If you set “MIDI I/O Mode” = Single, the Multi Part Performance can be set to respond to a single incoming MIDI Channel. If you set the Mode to Single and select “Ch.1”, for example, all 4 Parts of the “CFX Concert” will be able to sound as they would if you played the MONTAGE keys. Parts linked by the green KBD CTRL icon will all be able to respond on the Channel you select. You can trigger as many as 8 Parts, 1-8, with your single incoming Channel.
All non-KBD CTRL-linked Parts are unavailable via MIDI IN.

Hybrid
If you set “MIDI I/O Mode” = Hybrid, the KBD CTRL-linked Parts can be addressed on single designated Channel, and the non-KBD CTRL-linked Parts can be addressed on the Channel number that corresponds to the Part. For example, the 4-Part “CFX Concert” can be addressed on the MIDI Channel you designate, and the individual Parts, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 and 16 (in this example) can individually be addressed on the correspondingly number MIDI Channel.
Hybrid is a combination of Single + Multi.
Single - one Channel for all KBD CTRL Parts
Multi - means an individual Channel for each Part

2 What is the practical way to connect the external keyboard to part 8 and play the other 7 parts from the montage keyboard

Set the “MIDI I/O Mode” = Multi. This allows you to play the MONTAGE as usual (all KBD CTRL Parts will respond).
Place whatever program you wish to respond to the external keyboard, without the KBD CTRL in Part slot 8.
By deactivating KBD CTRL on Part 8, you can set your external keyboard to transmit on Channel 8 to address Part 8. The MONTAGE keyboard will not address Part 8 unless you directly *select* it.
To avoid *selecting* it return the cursor highlight to the [PERFORMANCE (HOME)] position where it highlights the Performance Name. Upper [COMMON] button will be lit.

Slider 8 can be used to adjust Part 8

3 Is it possible to get 2 separate montage sounds by splitting the external keyboard

If by “montage sounds” you mean Parts, this can be accomplished:
__in “Multi” Mode__ by setting your external keyboard to transmit on two MIDI Channels (if possible).
__in “Single” Mode__by setting your external keyboard to transmit on the designated “MIDI I/O Mode” Single Channel… then build your 2-Part split using the KBD CTRL feature to link them. (use this when your external keyboard only transmits on a single channel).
__in “Hybrid” Mode__by setting your external keyboard to transmit on the designated “MIDI I/O Mode” Hybrid Channel…then build your 2-Part split using KBD CTRL to link them. The MONTAGE keys can be used to play any non-KBD CTRL-linked Parts (individually) by directly selecting the Part, 3 thru 16.

i got it .. thanks a lot..

i have prepared a brass ensemble that plays part 1 to part 8 at the same time. Part 9 is piano, part 10 is ep and part 11 is clavianet. i can trigger part 9,10,11 separately with the 2nd keyboard. for this i just need to change the midi channel of my 2nd keyboard (9,10,11 ETC) . I think the midi multi setting is the only option for this preset. well for this preset the sustain pedal connected to the montage for the parts in part 9,10,11...16 does not work. is it possible to use the same sustain pedal in the sounds from part 9 to 16 as in my multi sounds between parts 1-8?
When I switch to the PART 9-16 screen, the sustain pedal is activated, but this time the part1-8 brass ensemble is disabled.

Controls for Parts 9-16 are turned off (keyboard control is forced off). You could split your sustain pedal and feed it into both Montage and your external controller.

The sustain messages have to come from external MIDI unless a Part 9-16 is selected. And then it would only be the single selected Part that would receive the sustain (also your other Parts in 1-8 with keyboard control ON would no longer receive any note on messages from pressing the piano keys - only the selected Part in 9-16 would).

Hybrid mode can work for this setup as well - but if there's no reason to have Parts 1-8 assigned to the same MIDI channel then Multi Channel mode is best.

additionaly when part1-8 is open on the screen, part 9 has a sound with assignment keys and superknob assigned, and part 9 is triggered by the second keyboard, can assignment keys and superknob be used on the yamaha montage?

can assignment keys and superknob be used on the yamaha montage?

The super knob can control any part.

You can NOT manually play parts 9-16 unless you first select the part but they can be played by a remote device.

thank you All ... .

[quotePost id=121131]

can assignment keys and superknob be used on the yamaha montage?

The super knob can control any part.

You can NOT manually play parts 9-16 unless you first select the part but they can be played by a remote device.

YES . Remote device can control the keys by changing remote devices midi channel (9-10-11-12....16)
As ı saw sustain pedal or other assignments are now working while a remote device (midi keyboard ) triggering the part 9 -10 or 11 ... etc and while the first 8 parts are visible on the yamaha montage monitor.