I use a MIDI keyboard as input, it work channel 1 only, and I use an external synth module as output.
Start method:
qjackctl
a2jmidid -e
ardour
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.
I suggest to only set the inbound filter per track, then. Since the track will always only receive the given channel, by induction, that also changes the output.
In any case you probably want a couple of MIDI tracks without instrument, routed to a MIDI-bus with the instrument. Then use Preferences > MIDI Ports > MIDI Input follows track selection