Here's some more info posted in Tips n Tricks for anyone else looking to use Halion:

https://www.dropbox.com/s/r9jbqkmbs28v409/Biab-Halion.mp4?dl=0
all tracks are re-route to the Utility #1 with Element and Halion.vst3

the Drums are sending to stereo 2 through FX
the guitar is sending to stereo 3 through FX
all other tracks are sending to stereo main
all goes to Biab Master.

There is also
Advanced Mixer for Biab Win/Mac