Speaking of secret bosses, load up one of the three complete saves (doesn't matter which character) and revisit the Land of Departure. That's where you'll find the traditional FM secret boss that turns out to be a character from a future game just like Xemnas in KH1 and Terra in KH2.
Ok, got through the double secret ending stuff. Loaded up Ventus, nothing happened. Trying with Aqua now, and nobody showing up there for a fight either...