Actually, I don't think there are going to be any summons in KH2.
I believe it's going to be replaced by a character combo command, where Sora executes a serise of attacks with a worlds character like Auron, Mulan and Jack Sparrow.
This was taken from OPM
Quote:
Combo attacks are similar to summons of other games: powerful moves with slick visuals where Sora teams up with a new party member and basically takes out anything in the surrounding area. For example, Sora and Auron perform a Bushido attack, which stacks several strikes before culminating in some crazy sword-whirlwind frenzy that slices everything around the two(the attack is performed by successfully timing Reaction command presses).