Cool to see someone using the Rev2 in the same way as I do (live performing, bass on the left "A" side).
Let me rephrase your question so you know whether I understand correctly: In split sounds, you sometimes want the upper keys sound (A) put out to your Kaoss Pad, and sometimes the lower keys sound (B) to your Kaoss Pad.
As far as I know, this is not possible: A is always right of split, B is always left of split; A is put out on the A(L+R) connections, B on B(L+R) connections.
(not entirely on topic, but by the way: Note that there are quite some different behaviors of the outputs available depending on which holes are plugged. For example, plugging mono cables in AL and BL gives you different mixes on the cables than AL and BR. Like many functionalities, this is not explained in the manual.)