...Why must KH crossovers suck.so.badly?
D:
Seriously. This is a series practically made for 'em. Just have Sora and co. land on the planet; no need for...alternate realities and random space-time portals (Although it would be nice if someone could actually pull that off).
No single planet or something in your choice, like
Dr. Who? Have 'em run into each-other out in space, or randomly run into each-other while on a planet.
Simple.