I've learned that by accident.

Solo one track, then unMute the other.

Same procedure in RealBand, but more confusing with the options there:
1. Solo more than one track, but the P/M button is only active for the first selected track
2. deselect the solo option for that track than the P/M button is no more active.

I have a topic on the wishlist some time back where I wish that BIAB and RB grow together. Make them similar in the essential interface regions for a "same look and feel". That is, have functions that are similar enough behave the same and their controls look the same possibly in the same location on the screen/in the floating window.

Desktop; i7-2600k, 8 GB mem., Win 10 Pro, BIAB 2017; RB 2017 - latest build
Laptop: i5-2410M, 4 GB mem, Win 10 Pro, BIAB 2017; RB 2017 - latest build