Mega crash uses two supers bars, it's just a way to get out of opponent's combos. You "burst" and it sends the opponent flying back.

Double supers you perform your normal super then press P (the partner) button straight after. We couldn't figure out the timing, it just ended up being a lot easier to press P along with two attack buttons at the same time eg. Ryu we'd do QCF + ABP. But that wasn't 100% consistent.

There's also delayed supers, you do one character's super then you follow up with your other character. This was a lot easier, you just do the super command of the character in play then do the super command of your tagged out character.

EDIT: Also where did you order Taiko no Tatsujin Wii from? I assume you got it online and not at a bricks and mortars store.