Played a few games yesterday, won about 10 now with all race combinations (2 zerg, 2 protoss, 1 terran 1 zerg, etc, etc,)

And so far this is what we have come up with:

Protoss: The most standard, do a cannon rush. Build probes until you got about your first chrono boost, but build an early pylon and then a forge in your base asap. move your pylon builder to the outsides of enemies base. Make sure they don't attack it until you got atleast 1-2 towers up.

Zerg: Get speedlings asap, and a queen. Hopefully you are not dead yet when it's done. Now you must have map controll, and then abuse their high speed. Attack and run attack and run. One of the player could attack the enemys army and run so that they will focus those speedlings instead while you or him/her destroy their workers/main building.

Terran: Marine-Bunker rush about 8-9 workers but send 1 early. Get orbital command.
If everything fails, switch to reapers ASAP with uppgraded speed and just spam them then do as with speedlings.

You can choose to deal with one enemy at the time, but if you are both protoss you should take one enemy each. It will be easier to co-op, but you will need to avoid the other computers army while you attack and run. Map control is there key then.
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(Usually you would want to take one enemy base each, but sometimes it doesn't work. Then try to go for the same enemy (the terran even had thors, but I lifted of my barracks and helped kill him instead after I totally failed when I salvaged my bunker to early :P. Then suddenly the protoss had carriers, and then A MOTHERSHIP and we both were like WTF. But he teched to mass void rays while I got mass reapers and just took map control with scan and "scouting mules" so I knew where is army was. Then I just took down the nexus when I could and he took down buildings when he could. So don't be scared by their sheer number aslong as you can get them where you want them.)