The only 2 on 2 fighting game that was done well was Tekken Tag Tournament for the PS2. I love this game. Hopefully Namco will bring Tag back after part 6?
The Dead or Alive series has had the Tag Battle option since part 2, but the gameplay has always been kinda bland.
Guilty Gear Isuka is also 4 players but the turn button really hurts the game’s fun factor. This game had a lot of potential, but with the Guilty Gear license now owned by Sega, I doubt will see an Isuka sequel anytime soon.
Tekken Tag Tournament
the games that come to my mind:
Tekken Tag
DOA (4 is best in this regard)
Smash Games
Power Stone
Gaurdian Heroes
um… oh yeah
GGI…
theres lotsa others… but i cant remember em… and most of em are no good…
smash brothers
Power Stone
Wrestling…
Since TTT was mentioned then how about SFEX3? I’ll also bring up Groove on Fight which has a tag system as well and the Saturn version has a 4 player mode.
Yeah, you could have two people on a team in EX3.
wcw vs nwo!!! woooooo
There was some fighting game back in the day that I believe was made by Data East…I can’t for the life of me remember the name of it, but it had two guys. One was a martial artist and the other was some big wrestler type guy. But it had 2v2 modes in it that you could play with another player on your side or compete against them I think. It was a fun game, wish I could remember the name of it. 
+1 on this.
You haven’t lived until you’ve tried a 6p match in Guardian Heroes.
the gundam vs games are 2v2
It saddens me that no one has made a Guardian Heroes style fighter for online current-gen console play. The plane shift system and method of control in the game was SO perfect, and plays loads better than Guilty Gear Isuka. Civilian Child VS Sky Spirit Battle FTW!
Panzer Bandit on PS1 I believe also had an up to 4 player battle mode. But it was a HUGE ripoff of Guardian Heroes, so that’s no surprise. It’s a rather good, High Quality ripoff though!
Sega Genesis also had the Yu Yu Hakusho fighter, which, once again, is the same style engine as Guardian Heroes VS mode.
Castle Crashers really tries to be like these games versus modes, but the “REAL 3d movement” of it totally throws off one’s ability to aim at their opponent, and sadly, this totally ruins the mode for me. If it stuck with the GH plane shifting style for this mode, I’d probably play it much more often. Tis a shame…
This right here. Gundam vs games are a blast. Unfortunately, the PS2 ports are only 2 player. The PSP ports are 4 player though, which is nice. And of course, the arcade version is 4 player.
So true. Whould love the see the disciples of hagar (Colt/gunlock and Biff/Zalazof) and scorpion/astro again
Def Jam: FFNY
I hated Castle Crashers, never understood why people got so ‘ZOMG BEST XBLA GAME EVER’ with it.
Also, why the hell is Guardian Heroes not on PSN/XBLA yet? It’s probably because it was a Saturn game and would be a nightmare to port/emulate, but still! I would pay good money for a nice port of that with online co-op.
Most fighting games by Treasure, starting with (and all pretty much based on the engine for) Y? Y? Hakusho: Maky? T?itsusen for the Sega Mega Drive/Genesis which later evolved into the Bleach games. Also special mention goes to the fighter/beat 'em up hybrid Guardian Heroes for the Sega Saturn (as mentioned by MrMojoZ and wip3outguy7), pretty much a beat 'em up but using the multi-plane fighting layout of YYH (not the GBA sequel though, which was a more traditional beat 'em up).
Here are some videos.
YYH:
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[media=youtube]Qr8DW0dVAUg[/media]
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Guardian Heroes:
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[media=youtube]AsLYHpetUUA[/media]
[media=youtube]jf94wSankE0[/media]
Bleach on DS:
[media=youtube]8vnRjD_8hv0[/media]
[media=youtube]wUFkG2UlJTQ[/media]
3s
…played on the mugen engine
lol j/k please dun flame me. personally i don’t think 2vs2 game modes should be taken that seriously, you might as well load up some broken-ass shit with mugen for kicks. in 2vs2 mode for mugen, practically everything you do is an infinite.
[media=youtube]VdXmJZ_YPMQ[/media]
Vanguard Princess.
Sorta.
Street Fighter Alpha/Zero 3 on the Dreamcast had a mode where you could play 2 vs 1. Yup, 3 players and fighters on the screen all at once! Shit was so hype.