My Terran partner and I had two games in a row against the same two Zerg players, and they creamed us both times. Sure, we drew out the game, felt like we were doing well, but they either out-expanded us or out-tech'ed us in both games, leading to a loss.
Despite repeated aggression (but no harassment sadly), we weren't able to match them on macro. The second game, we prevented them from expanding to more bases than us (and for Zerg, that's pretty good), but they proceeded to stomp us with Ultralisks off of 2 bases each. Strangely, no mutas that game.
What lessons were learned? Well, Zerg certainly have mobility, and can harass you pretty much anywhere on the map with 12 Zerglings. Double-Zerg is doubly dangerous because they could either both 10-pool you and crush one player, or they could both macro hard. I think the trick is for one of them to do the 10pool and then both macro hard, scouting enough such that an army can be spit out if the enemy approaches.
I think their weakness would have been a marine drop, or even just repeated drops before Mutas were started in quantity. Honestly, I don't know how to beat that.