I've been having a lot of success as Zerg with a Terran partner in 2v2. There are some pretty sweet synergies that Zerglings or Roaches have with marines. Generally, Roaches are better, because their movement speed is similar to marines, and because they soak up damage while marines deal it. However, Zerglings can work well because it's nearly impossible to get through a mass of Zerglings to get at the marines, even with other Zerglings. The whole thing falls apart when one of your Zerg opponents make banelings, so after the first push with Zerglings, switch to Roaches (or get banelings yourself, if you like baneling wars. I don't, personally).
I really like the Infestor, as well. With a range-heavy ally like Terran, stopping your opponent's army in place with Fungal Growth while getting a great concave yourself is crippling to the enemy (not to mention the Fungal growth damage itself).
One thing you may think is good, but we've beaten a ton of players who do it, is when facing a Zerg player who masses Mutas. While a lot of mutas can destroy your economy, we've won at least 3 games where our economy was heavily damaged, but the knowledge that at least one opponent invested in harassment forces instead of a ground army allowed us to go stomp them both flat. In our last game, I pretty much lost my main base to Muta damage (especially after stupidly not building spore crawlers despite his wall-in with Spine Crawlers), but we simply marched in and wiped out their bases. They couldn't do the same to us because marines and turrets would have ripped up the fragile harass forces had he proceeded to my Terran ally's base.
In the words of Day9... "just go f-ing kill him!".