For those of you just starting out, consider stocking up on Drop-Me-Not items early on and using them to play through each world with each character in one go. (For example, Sora's Traverse Town, then Riku's Traverse Town, then whatever world you choose next with Sora, then that same world with Riku, etc.)
I actually ended up enjoying the drop system, in that you get to get to play chunks of each campaign instead of the whole thing at once. The way I was doing it though, was alternating between levels trying not to play the same world with both guys. I ended up doing The Grid with both guys and the similar stories/shared areas got a touch confusing. I guess if the drop animation wasn't as fast as it was it'd probably be annoying.
Although I think I might use this method when I unlock critical difficulty (with the way proud's kicking my ass, this scares me though)