If you have written a number of separate MIDI parts into the Soloist Track then it will always retain that structure in BIAB with individual sections separated by the Instrument codes. Save in BIAB and open in RealBand. Highlight each separate instrument section in the Soloist Track, Cut and Paste to an empty track at the correct time starting point. You will then have all the parts separated onto individual tracks and can set differing Pan, volumns etc for each.

You can, also Drag/Drop the Soloist Track into RealBand, but you still need to separate parts manually.