I think you probably play as more than just Sora. I bet you play as Blond haired kid at the begining. They probably run on the same character phisics and skeletal engine in the game, like the difference between blue and red flying heartless. You know, very similar, so perhaps, dual playable. Anyway, I say it's one player, many characters, like in COM.
Wouldn't in be great if you got to play as the summon that you summoned. Imagine being Sora, and then suddenly, you trans up and magic up and then you're cloud with the buster sword. It would be really great. Yeah.