Imagine a battle in which you have no partner but are forced to fight two adversaries. Now imagine those two adversaries take shots at each other while fighting you. Imagine that if you're taken out first, the battle ends but the winner is decided instantly from among the survivors and that you can still win in you had placed a bet on the winner, kind of like they were your second choice of hero. Maybe that last part's a bit far-fetched. One-on-one battles are so played out. Tag-team battles are old-news too. They're still just o-o-o with a dash of character swapping.
But Dissidia's dash-around combat system would allow for multiple adversaries. It would make sense to take on Cecil, Kain, and Golbez all at one. Maybe it wouldn't make so much sense to pit Cloud, Sephiroth, and Tifa against a player, but whatever.