I had an epic 2v2 game yesterday. I'm Protoss with a Terran ally and it's versus the same combo... so essentially a mirror match.
We get attacked 8-9 minutes into the game and at this point in time I'm warping in Stalkers. The Protoss has more stalker than I do, and my ally's marine army was just wiped out for some reason and just leaves the game. Luckily, the two players decide to go into and wipe out the rest of the Terran base and leave me alone for about 2 minutes. In those two minutes I warp in Dark Templars and wipe out the attacking army, and with the additional minerals given to me by the Terran player who left, I built a whole bunch of cannons blocking my ramp entrance.
The opposing Terran player's second army is a large group of Siege Tanks which start wiping out my cannons. I use DTs to wipe the tanks out, the Terran scan, but a 12 second scan isn't enough to stop 6-8 DTs attacking his tanks. At this point the opposing Protoss player lags out.
Now it's on -- 1v1. I use my DTs and attack the Terran. He has no detectors and I wipe out his base. He takes control of the Protoss player's units and has amassed Void Rays. Using the DTs I wiped out the Protoss mining operation as well, so all he's got is an army of Void Rays and a whole bunch of cannons built. My small army of Stalkers and DTs aren't a good match against void rays.
During this time I secretly made an expansion and had amassed a fair amount of minerals so I built 9 warp gates nearby and warped in about 15 zealots while the Void Rays were slowly destroying my base. I use my zealots and take out the pylon powering the cannons and wipe out the rest of the base and win the match.
That was probably the most interesting 2v2 match that I've had. When players disconnect and leave, their mining operation and whatever minerals and units they have can be controlled by the remaining players (on the same team), so it can lead to interesting situations.