Thanks everyone!
Turned out i was able to use V25.
So Brian-bmw's trick worked on me (2003 E39, MKIV, 16:9 display, BM53 radio) by downgrading from V27.1 to V25, to restore the split screen when an audio source is selected. Whew... And upgraded from V25 to V27.1 without loosing that split screen feature.
I'll add more details to following thread,
http://www.bimmerboard.com/forums/posts/245743
Thanks much!