after re-reading what I wrote above, if you're starting in BIAB, it would probably be easier to just export your expanded track as audio, then start a new RB project at half the BPM and import the expanded WAV. That way you don't have to enter the chords one way, generate, then delete half the chords in the same project.

This is another reason why RB is a good DAW companion for BIAB... you can keep loading and generating tracks after BIAB did its part. That adds options you wouldn't have with other DAWs

https://www.dropbox.com/sh/ey71h8ep047758h/AAC12jNM0Hz9Q8zsh5iDeqNHa?dl=0