I have an accordion double staff, I have assigned top staff to MIDI channel 1, bottom to channel 2. When I play on the keyboard on the bottom staff it plays channel 1. Playback, however, works fine i.e. it’s the MIDI Thru that’s not working.
The choice of instrument for MIDI auditioning works at the instrument level, not at the staff or voice level, so this is how I’d expect Dorico to behave at the moment.