Indeed. otherwise it sucks for whoever plays as the toads. If there's only one toad, he's an actual character. if there's two, than they just feel like filler.
Mario and Wario would never team up in the same game. Unless they're being forced to in Mario Party 2-on-2 minigames.
There could be some kind of story where Mario needs Wario's help and Wario sees an opportunity to collect fat cash while helping him, even though they are both hesitant to trust each other. In single player there are different levels for different characters (maybe you play as Luigi etc. too?) and after you beat the game you're able to replay all those levels with different characters, using their unique abilities to get bonuses etc. Anyway, on some levels, you'd play as one character and try to undermine the other character's efforts (i.e. blocking a path with a boulder) and in the next level when you play as the other character, you must overcome you're own obstacle.
See, I just invented an entire AAA Nintendo game for free.