As far as the 1 person, 2 character thing goes, you'll likely control the attacking character with the c-stick, so you'll be able to shoot in any direction. much better than simply "shoot in front, hey, where'd that item come from?"
Also, you'll notice that one of the screens has a different-colored peach. That means that there's certainly different costumes but, perhaps more importantly, you can play as the same character as someone else. Finally!