Is there a way to “hook up” Common assignable knobs 7 and 8 to respond to cc23 and cc24?

Press [UTILITY]
Touch “Settings” > “Advanced” > Set the “MIDI I/O Mode” = Single or Hybrid
Select a MIDI Channel on which you want to communicate with the MONTAGE Performance (Common).

Here’s what’s happening:
When you select Part 4, because you were in “MIDI I/O Mode” = Multi, the CC# would address the Part Assign Knobs as you correctly suggest.
By MIDI convention, CC# are always Channel specific messages. They are sent on a specific channel and the message is only for that channel. (This is why you must *select* a Part - in doing so you are selecting the Channel for the message).

When in “MIDI I/O Mode” = “Multi” each MONTAGE Part is set to a correspondingly numbered MIDI Channels… there is no Common MIDI Receive Channel when MIDI I/O Mode = Multi

The 8 Part Assign Knobs for Part 1, use CC17~CC24 on Ch 1
The 8 Part Assign Knobs for Part 2, use CC17~CC24 on Ch 2
The 8 Part Assign Knobs for Part 3, use CC17~CC24 on Ch 3
The 8 Part Assign Knobs for Part 4, use CC17~CC24 on Ch 4
…and so on…
The 8 Common Assign Knobs send and receive Sysex messages, via the Knobs…

Like the Super Knob, the Common Assign Knobs can address any or all Parts (using Sysex).

Set the MIDI I/O Mode = Single or Hybrid
However, when you use “MIDI I/O Mode” Single, the entire Performance can be controlled from an external device that is transmitting on a single channel. When you use “MIDI I/O Mode” Hybrid, the KBD CTRL linked Parts can be controlled from an external device.

The Common Assign Knobs can be used to control any parameter assigned to a Part’s Assign Knob.
What this means is, say in your Part 4 you assign a parameter to Part Assign Knob 1 (Part Volume) when you go to the upper Common/Audio level… add “+” a Control Set; Set the Source = (Common) Assign Knob 7; direct it to “Part 4 Assign 1” as Destination.

Now when your external controller sends In CC23 it will manipulate Common Assign Knob 7 which, in turn, manipulates Part 4’s Assign Knob 1

Single/Hybrid allow you to access Common Assign Knobs on a basic receive channel you select. The CC numbers will apply to the Common Assign Knobs

Cool.

Why would I choose Single vs Hybrid in this situation? Just a matter of controlling one vs many Parts via Kbd Ctrl settings? Anything else?

Cool.

Why would I choose Single vs Hybrid in this situation? Just a matter of controlling one vs many Parts via Kbd Ctrl settings? Anything else?

MIDI I/O Mode = Multi — All MONTAGE Parts are on separate MIDI Channels. Means multiple Receive//multiple Transmit Channels. The Channel # corresponds to the Part #.

MIDI I/O Mode = Single — All KBD CTRL Parts are on a single, selectable, Channel. Single means a single channel is used to Receive and Transmit - allows all KBD CTRL linked Parts work as a group. An external controller that transmits on just one channel at a time can play MONTAGE’s multiple Parts, simultaneously..

MIDI I/O Mode = Hybrid — This is a combination of the previous two I/O Modes (Multi+Single). The KDB CTRL linked Parts are addressable on a single selected Channel; and all other non-linked Parts are behave like multi; they receive and transmit on their correspondingly numbered channel.

You would use Hybrid, instead of Single, if you wish to have all the non-KBD CTRL linked Parts available as separate (Single Part) tone engines. If you select a MIDI communication Channel that is already in use, an ❗️ exclamation mark will appear to alert you to the conflict.