During playback assign 2 sounds to a single instrument (instruments.xml ? ) in MS4 or MS3

Hi.

What I am finally trying to achieve is to record a good mp3 choral practice recording from a choral score.
I already have this thread: https://musescore.org/en/node/345223
Ideal mp3 choral practice recording has a metronome, target (say, tenor) part playing loud, and other parts playing quiet. This can be easily achieved in the mixer in MS4, no problems; just reconfigure volume levels and export 5 different mp3s (one more for "balanced voices").

The problem is with sounds, sound fonts. Default MS Basic choir is especially bad (for this specific goal), as well as Muse Sounds choir. The main problem is that during the very beginning of every note, both choir options I tried have quite a slow pickup, which leads to the choir part literally lagging behind the metronome significantly in faster passages. We need some kind of striking instrument instead.
Sufficiently articulated striking is also important for the vocal phrases where several notes are in the same pitch in a row - mp3 choir practice needs to reflect rhythm, and it needs to be heard well.

I have found that Grand Piano from MuseSounds sounds beautiful, comfortable to sing along, have great striking capability, and does not lag behind the metronome at all. But pianos cannot sustain, while the choir has to sustain from time to time. The piano is only suitable for fast passages.

I have also found that either Violin solo or Viola solo from both MuseSounds and MS Basic have the decent striking capability (not as good as Piano), also can sustain, and also are a pleasure to sing along.

I have tried "New Age Synthesizer" as advised on another thread, but I found it strikes lacking (way behind strings) and also not pleasant to sing along.

Why not get the best of 2 worlds, and make MuseScore playback every choral voice in both Piano and Viola solo simultaneously? I have tried this, and it sounds really great!
The only problem with this (without creating custom instruments with custom sounds) is that for every part I will have to maintain 2 independent staves with independent content, I will have to copy paste Tenor party into Piano instrument and into Viola instrument, and if I have to change the score itself, I will have to maintain at least 2 copies of every party at all times.

Ideally, I would want to learn to create a custom instrument, "TenorPractice" that would look exactly like regular tenor on a score, but during playback sound exactly like Piano + Viola combined. This would allow me to maintain a single score file (both for printing and for mp3 practice creation) with a single copy of information (no double staves with copy-pasted content).

I have tried to manipulate instruments.xml of MS4, and failed to achieve any kind of change in behavior, even when deleting channel information from the Tenor instrument section altogether (while applying MS Basic sound font in Mixer).
If this is actually impossible to achieve in MS4 but achievable in MS3 I am willing to downgrade (though I already saved a bunch of new scores in MS4 and I am not sure if any conversion is possible).

That might be not the only way for me to reach my goal, and I am willing to consider other ones, including "just maintain 2 copies of every party at all times or recreate them whenever you need to export an updated MP3 and then delete them", but this is so blatantly inefficient it makes me cry.