Hi hammer,
I have the same setup in the store (a800pro) but I do not have a problem. I use master keyboard - one channel, and only use the upper zone on the a800 on channel 1. This works fine for me with split on or off. Check what is happening with the arranger mode. It may be forcing you into pianist mode.
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Roland G-70, FP-4, BK-7m, SonicCell, KC-350 x 2, DS-5 x 2, A300-Pro, Sonar X1 PE, BR1600CD
Yamaha HX-1 System 1, KA-20x2