Main thing is can it be done

Yes.

how do I connect to and configure the Montage to play the Montage sounds while being able to play a different sound using the Montage that is triggered only by playing the Montage keyboard?

Bad Mister explains it in his reply in this thead - thoug you may want to read the entire thread
https://www.yamahasynth.com/forum/2nd-keyboard-control-from-montage-midi-in-connection#reply-121097

Bad Mister's explanation in this Modx thread applies to the Montage as well.
https://www.yamahasynth.com/forum/does-modx-support-connecting-multiple-midi-devices-to-it#reply-120111

Thank you very kindly Andrew. Much appreciated.

Kurzweils are good controllers for Montage. I no expert on every Kurzweil model - but most allow to address 16 simultaneous zones for external control with different MIDI channels. This is great because you can leave the Montage in multi-channel mode as this gives you the best flexibility when using an external controller.

The Montage is arranged into 16 "Parts". Each Part is on a different MIDI channel (abbreviated CH here). Part 1 = CH1, Part 2 = CH2, Part 3 = CH3, ... Part 16 = CH16. These are not flexible - that's a fixed relationship between Part number and MIDI channel for Montage's MIDI receive while in multi-channel MIDI mode.

Any Part with keyboard control turned ON will, among other controllers, allow for the local Montage piano keys to control this Part. Parts 9-16 cannot have keyboard control turned ON so these are best for external control in your setup. There's more options of local control with Parts 9-16 but they can only be locally controlled one Part at a time with the local piano keys -- which normally is limiting. However, for your case the Kurzweil, I would suggest, would have Parts 9-16 to "play" with and Parts 1-8 would be those Parts you want to "play" with Montage's own local (piano) keyboard.

There are sounds that require more than one (multi) Part to fully represent that sound and there are some that are single Part. There are some pianos that are condensed and use only one Part (such as "CFX Stage" ) and some that take up a bunch of Parts (like "CFX Concert" ). If you wanted to control a multi-Part sound then you would place them in say in Parts 9, 10, 11 (for a 3 Part sound) and then have the Kurzweil send the same note on/off to MIDI channels 9, 10, and 11. It's easier to deal with single-Part sounds where there is a 1:1 mapping between the single sound and a single MIDI channel.

Note that you could control more than 8 Parts with the Kurzweil if you wanted. You're not limited to 9-16 that all have keyboard control turned off (and no way to turn it on). Any Part 1-8 can also turn keyboard control OFF and then you could use this Part also as an externally controlled Part using the Kurzweil as a master controller.

I could be wrong, but I doubt you would ever need more than 8 Parts to be controlled by the Kurzweil. This simplifies the worflow because you can always designate Parts 9-16 for external control (Montage as a slave) and then Parts 1-8 are all free for local keyboard control.

As a next level possibility (you didn't ask about this) - you could route the audio output of either of your other keyboards to the A/D Inputs (L/R) on Montage and then utilize the Montage's effects so these effects are applied to your external keyboard sound and then routed out the audio output (Main L/R) of Montage. The keyboard you setup like this doesn't need its output to go into a mixer or anything else because the audio from this keyboard would route through Montage's built in mixer (with effects). Effects are optional, so just employing the mixing and keeping the signal dry is a possibility too. There's an A/D gain knob so you can control the relative volume of the audio connected keyboard using this knob.

Thanks very kindly for your considered reply Jason - much appreciated. That is extremely useful information. I'll proceed to purchase on the basis of your's and Andrew's response.