Hi ,
I’ve got my p121 and Sx900 stacked in a similar fashion ( don’t have pedals, I don’t play organ), the only thing I can think of, is , with a setup like this , you still have a way bigger gap between the 2 sets of keys than you would with an organ.

Maybe it doesn’t really matter, I don’t know, because I only use the p121 as controller for the sx, I don’t play both sets of keys at the same time.