There is no absolute need to pan your instruments within the stereo field. You can - that's up to you - but a split would mean low notes from the bottom piano key to some key near middle C (or wherever you want the split point to be) will play one set of sounds and anything from the split point up will play the other set of sounds.

It sounds like you want the right hand to have flute+violin+synth. All at the same time (layered) - or one of those (flute OR violin OR synth) at a time?

This video shows creating a split. https://youtu.be/WIumqa4bEZA?t=97

or this one which shows more modern firmware:

https://www.youtube.com/watch?v=uy23zJWqXpE

Paul, as Jason says, you can layer sounds without panning. In that other thread, you asked about layering two sounds "with distinct clarity" and I suggested that you can increase the distinct clarity between two layered sounds by placing them at slightly different locations in the stereo field, (which also obviously requires some kind of stereo amplification). However, it is not a "must."

You do not typically create these sound setups "in real time on stage", especially on a board like a Montage. You set them up in advance, and assign the entire combination to a single button for instant recall during performance.

My interpretation of what you want is a little different from Jason's... you talked about there being two instruments sounds in his scenario, Jason sees four, and I think the right number is three. 😉 I believe you want a Rhodes sound on the left of a split point (one sound), and a layered flute and violin (two more sounds, which you are referring to as your "synth" sounds) on the right. On the Montage, you would create a 3-instrument Performance, and for each of the three instruments (Rhodes, flute, violin), you would specify the desired lowest note, highest note, and stereo pan position along with any other parameter you might want to tweak (relative volume of each sound, any effect you might want to use, etc.), and save that as a User Performance for instant recall when needed.

If you like to assemble your sound combinations on the fly rather than in advance, some instruments are more geared toward that way of working, like the new Yamaha CP73/CP88, but that doesn't happen to have a flute or violin sound. The Montage (or MODX) is much more versatile, but you will have to put a little more time into finding ways to quickly mix and match sounds. You could also look at some non-Yamaha boards that have a more "direct" approach, like a Nord, or Korg Grandstage, or Roland VR-730. No matter what, though, there is a trade-off between having lots of quick, direct control vs. lots of deep customization. (I don't think any of those could give you Rhodes on bottom and layered flute/violin on top, except for the Nord Stage 3, and I'd say even that won't give you Montage calibre flute and violin.)

Usually if one cannot clearly articulate - even in generic terms - what they want, it's going to be difficult to configure your keyboard to accomplish your vision.

This would include:

Key ranges on keyboard where you want each sound (including lowest and highest note - either using MIDI notes or "octave above/below middle C" orientation.
If you want sounds that share the same keys to be layered (playing at the same time) or to be soloed (audible for that particular sound without any other sounds using shared keys sounding). Or some combination like "piano+strings in right hand but not synth in right hand" while at another time in the song "synth in right hand but not piano+strings".

Nah, you don't need to know where the split will be located to be able to tell someone how to create a split. You would need 87 separate sets of instructions for an 88 note keyboard. 😉

I think they only thing that really threw you was his use of the word "synth." I think his calling flute and violin "synth" sounds comes from boards which categorize sounds this way. Some Rolands and Nords have these kinds of sounds in their "Synth" section, similar to how Yamaha CP73/CP88 and Kawai MP11 put them in their "Sub" section, or Korg Grandstage has them in its "Ensemble" section, some boards just use these terms as broad categories for access to different kinds of sounds. Read his post from that perspective, and I think it's clear that he simply wants Rhodes on the left side of a split, and layered flute and violin on the right (i.e. those are the "two synth sounds" he is talking about), then it all makes sense.

I'm simply advocating thinking in terms of how the keyboard relates to defining splits and layers - using ranges of notes. You need to be comfortable with a "bottom" and "top" note concept. I don't need to know the exact notes to convey the mechanics of where to go and what to do - but the user needs to be able to articulate this stuff in order to be successful when programming the keyboard.