It seems like each patch should be a separate instrument, with independent instrument switching for left and right hands. But the keyboard instruments all seem to automatically take both clefs, and you can’t assign two of them to the same player at the same time. I’ve checked the documentation and forums and apologize if I missed something.
There isn’t a good way of doing this at the moment. I believe that you may be able to achieve independent playback from a fixed splitpoint: in Halion Sonic SE I think you can have two slots listening to the same input port and then you can set a pitch range filter so that only notes above or below a given pitch play back on each slot. In the future we hope to support independent playback for each voice which should give even more control than just routing staves separately.
I’ve just used two voice staves with two players. Unless the score only uses a few different patches, I find it unwieldy to get any working playback, so just use shift-x text.
I think I may have stumbled on a workaround - under “Electronics” (not keyboard) there’s a “sampler” instrument which is just the treble clef and a “synth bass” instrument which is just the bass clef. So create a group, with separate left and right hand players switching between multiple instances of those instruments, each of which can have its own text label and patch for playback.