This was asked for sometime back so you could do what you need to do now, but PG is very slow to catch on and never thought you would want to do that, so it does take a lot of nagging.
Here's an example of it if you watch all the video below (Chords&Tracks Tab) you will see you can select to send WAV or MID (chart) then select the channel you want it sent on.
Rustyspoon# New GUI Example Add-ons

To do it now you can use a midi re-channel plugin after the BiabVST or on a track you send the midi to.
You can either solo the midi tracks one at a time, or just use another instance of the BiabVST.

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