I have track 1 stereo mix of song set to Stereo ch 1-2
and Direct Input Multiriff tracks are all set to Mono ch 3
Set the BB Plugin track to 16 channels
create a send on ch 3 to the Amplitube track

https://www.dropbox.com/s/xfhpeaynjmpwloy/BBvst-Reaper-Send-Channels.mp4?dl=0
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ReaTrak Biab & Plugin in Reaper
Free Biab Chord Picker Tool