Oh, I'm sure.
I'd be willing to bet you can merge with other Disney characters. (Mmmmmm, Simba...)
Well, one, Sora gets to use two keyblades, gets lighter hair, and one keyblade seems to be color-wise the opposite of the other. Seen the KH2 Sora statue?
I have another theory on the BHK, but I'm not sure if it's absolutely correct since I haven't played KH1. I'd like to draw a picture first, though.