How do I counter double-terran in 2v2 with zerg?

Solution 1:

What you can do is get Infestors. Several of them would be ideal. When the MMM mass bunch up, drop fungal growth on them. Then burrow your infestors so they don't take damage and spam infested marines as quickly as possible (they're cheap energy so several infestors can create a mass of them quickly). Also, burrowed infestors can still spawn Infested Terrans. This is also assuming you have other units with your infestors. Roaches get kited pretty easily with Mauraders but if you use fungal growth that won't happen.

If they're a heavier Maruader build with few marines then Mutalisks will be the best counter. But don't relay heavily on them after you take out the first wave of Marauders because a smart opponent will switch to heavy marines and vikings if he sees you've gone mutas.

Solution 2:

Marines in SC2 seem to be almost impossible to beat when there are enough of them, especially with marauder support, so your best chance is to destroy his supply and his reinforcements with drops, mutalisks or nydus worms while slowly killing his troops with suprise banelings (don't forget they can burrow, quite deadly if they jump out in the middle of a ball of marines) and hit-and-run tactics. Infestors work quite well, too, but if he has enough medics it will just slow him.

An MMM Ball moves relatively slowly so harassing his workers with some mutas and killing the marines that move from his base to his main unit ball should work quite well to prevent him to get enough reinforcements to fill his losses. In the worst case he has to keep half his army in his base to prevent your harass, which gives you an significant advantage at the front.

I do not recommend a frontal assault, even if you have a big ball of units yourself. It will cost a lot and then all of you just have to fill your losses and repeat from step 1. It's cheaper to hit-and-run, even if you have to sacrifice an expansion, just make sure you always have more expansions than your enemy.