Short answer is become adept at scenes. Even though for Montage - it translates fine to MODX. Building custom scenes, etc: https://www.yamahasynth.com/montage-category/mastering-montage-splits-layers-scenes-and-articulations

I previously owned a MO6 (before "MODX" ) and used sliders. I would do a "flip" action where I grabbed both and slid one down while the other slid up. That was common. Now I no longer touch the sliders because the SCENE feature (4 lit scene buttons) covers this for me. I setup scenes with the mute properties or volume levels setup (memorized) for me. When I push one of the scene buttons, it recalls the memorized setting(s). Since scene buttons are mapped to a CC - the right kind of MIDI pedal can be used to link the foot pedal switches to MIDI messages that "push" the scene buttons, one at a time, for you.

The latest firmware even adds a new possibility - to have scenes memorize keyboard control. This allows for you to hold an old sound you're switching away from (say organ) and seamlessly bring in piano. After you press the scene for keyboard control changes - the piano, for example, would have keyboard control turned ON and therefore any new left-hand piano key mashing would sound as piano. Anything held before/through the scene change would continue to sound as organ until I let go of those left-hand piano keys without being interrupted (and organ's keyboard control would be turned OFF by the scene change).

I haven't had the new feature - so I've always been able to get by with using mute.

I'm not listening to the song and do not know it by heart - so just looking at the score coupled with my tune ignorance here - I can't tell what the measure 5 bass clef is supposed to be. I'm guessing it's organ from your description.

Looks like Harmonica doesn't need to be switched on/of because it's key limited. Whenever the synth plays the D a 2nd above middle C or lower, or plays higher than the D an octave above that - it would be playing alone because you can note limit the harmonica not to layer for these ranges.

Looks like you have to cut off the synth in the coda (at least it looks like organ only). And you've got a measure and 3 beats to do this - so muting would be fine (no need for keyboard control).

NOTE: for memorizing muting in scenes, you need to turn ON the Mixing memory. The scene setup menus show a number of categories for MODX to memorize and can be turned on or off (to memorize or to leave alone when the scene is recalled). So be sure to turn on Mixing. Specifically, "Mixing 2". Honestly, I usually turn on mixing 1 and mixing 2 together - but for just mute - mixing 2 is all you need.

So one scene would be just for turning the synth off (your "last" scene).

I never see left hand piano labeled in the score. This is a matter of you knowing the tune more than me and not spelling it out in the score.

... but because it looks like sometimes piano or organ is layered in the right hand and sometimes it's only supposed to be left hand - I would add two PARTs to split the right and left hands of organ (and maybe piano - if there's any left hand piano as you wrote) so I could mute the right while leaving the left sounding.

... also, I'm not sure how you write your scores. I mostly write scores as how I play the notes - not as they sound. Sometimes bass clef parts end up being high-high notes because there's mapping that puts them there behind the curtains. So it's not readily apparent if you have another way to keep the parts from layering (like using a different keyboard range for the organ and/or piano PARTs vs. the synth/harmonica). Sometimes just using keyboard range can keep you from having to mute/unmute or the slider acrobatics.

Short version: I'm a new MODX owner, and I'm looking for a way to quickly alternate parts. I'd like one quick action I can perform at a gig to mute two parts and unmute a third, and then to flip back to the first two. Is there pedal control or quick switching? Right now, I'm using the sliders but it's an inelegant solution.

There are several ways to switch instruments with “one quick action”.

Doing it with sliders requires multiple moves and is not as quick as one button press.

Method 1
Using the [KNOB POSITION 1]/[KNOB POSITION 2] buttons you can switch the Super Knob instantly from minimum to maximum position (default).
Set the first condition to Super Knob is at minimum, the second instrumentation when Super Knob is at maximum.

Assigning PART VOLUME of your instruments to the Assign Knob/Super Knob system will allow you to play one combination of instruments and then instantly switch to another; the exact Volume balance can be stored.
Assigning the PART VOLUME parameter to an Assign Knob position can be setup by programming the Control Sets per Part.

If you need help with this assignment, let us know.

Method 2
KBD CTRL status can be memorized per Scene. Each of your eight red [SCENE] buttons can memorize both which Parts are active and at what precise volume setting they are set to.
From the HOME screen:
Touch “Scene”
At the very top recall “Scene 1”
Activate the SCENE MEMORY Switch for “KBD CTRL” and for “MIXING 1/2”
First, KBD CTRL: activate the KBD CTRL icon for the Parts you want initially active, 1-8
Next, MIXING 1: set the Volume slider to set the balance you require.

Select “Scene 2”
Activate the SCENE MEMORY Switch for “KBD CTRL” and “MIXING”
make your settings.

Now the Scene 1 and 2 buttons can be used to toggle your keyboard setup.

Hey! Not really on topic Stefan and I can't assist you with the MODX but I wanted to chime in and say cool! I do a lot of the same kind of split performing, just with different genres of music, and I think the MODX is a killer stage keyboard to use for multi voicing covers. That song looks like quite a task!

Hi all, thank you for your quick and detailed answers. I was able to get the Scenes working for the task, thanks for the suggestions!