I use a MIDI keyboard as input, it work channel 1 only, and I use an external synth module as output.
I make 2 MIDI track (in synth module I set channel 1 as Piano, channel 2 as Bass). In Ardour in the selected track I set inbound “Force all channels to 1 channel”, and set playback “Use a single fixed channel for all playback” to channel 2. I set MIDI input to MIDI keyboard, output to synth module.
If I put a key in piano roll then synth module play the sound with Bass.
If I put a key in MIDI keyboard then synth module play the sound with Piano.