I'd like a link to this interview sense it was stated that switching would be more random then that at one point. (This seemed like a gameplay nightmare and mis-translation at the time to me though)
I had predicted that the drop gauge would measure time between switching characters back when the trailer was revealed based on the raining soras scene. I also the idea for a the World Ends With You style battle system in which the same enemies would exist in different areas and the player would beat them by alternating characters.
this linked enemy idea was based on the new logo that resembles two hearts linked together.
It could also be that there are 2 completely diferent enemies that are linked together and if both riku and sora don't defeat them they will both fail, building on the trust theme.
As far as saving goes. I don't think it will be a big deal if you save with characters in different worlds. The game would just save both character's positions.
My issue is how they would handle cutscenes. It would feel really awkward switching between cutscenes about quazimoto and ones about mogli, but I suppose there could be battle drops and story drops that are separate from each other. story drops just happening at times when you switch primary character and battle drops happening in the middle of battles.
What happens after the story is complete though?