Sixfortyfive
He who pursues two rabbits gets two rabbits.
I think 3D is a better fit for it. I remember playing 2D tag team survival matches on MUGEN and those were chaotic as hell.
What Treasure games are tag team fighting games?
Yu Yu Hakusho Makyo Toitsusen
Guardian Heroes
Bleach DS 1 & 2
The Bleach games are the most competitively-sound. Guardian Heroes is an utterly broken mess, but also a lot of fun. YYH is a bit too old and clunky. They're all built upon the same basic gameplay though.
The main reason why these games work is because they forgo any notion of "forward" or "backward" facing attacks. Quarter-circle-left + punch would always throw a fireball to the left, and quarter-circle-right + punch would always throw one to the right, regardless of where the other players are positioned. A block button is added so that being stuck between two opponents isn't flat-out murder, but the mix-up and movement potential is good enough to still make offense potent.
It's a lot more elegant than GG Isuka's baffling turn button, and it's not half-assed like Capcom's dramatic battle Alpha modes or whatever SFxTK had.
The Bleach games had a decent competitive scene for a while. First 3rd party DS game with online play, too. I remember playing against Marn and some other people on IRC way back when.