2v2: ZP/T vs TT - how to not fall behind

Barring scouting some kind of all-in rush coming from the terrans, you should probably be expanding before the 26 food mark. Personally I would recommend 14 pool, 16 hatch, and getting two gas geysers up at ~21 food. If you want to play it safe though you can go 14 gas, 14 pool and get the hatch around 24. If you want to be a little risky for more economic gain, go 14 hatch 14 pool. The larger the map/distance between you and your enemy the safer the fast expand is. I would generally recommend getting speedlings with your first 100 gas, and a baneling nest with your next 50. Banelings are never a bad thing to have vs terran.

As for mid-game, muta-ling is generally superior to hydra-roach in team games, as the bigger maps makes the mobility and map control provided by the mutas and speedlings more powerful.

If you are falling behind, you likely are not building enough drones early. Drone hard early game, keep lings at the towers and one outside their ramp to spot when you need to start pumping units like crazy. With two queen boosted hatches you can get a lot of lings out by the time they walk across the map.

Personal Credentials: 1200+ diamond 2v2 team with both of us playing random.

Check How To Play Zerg in Team Games, accompanied with a video.

Early Game: Speed Zergling
Middle Game: Add Infestor and/or Hydralisks
Late Game: Ultralisks and/or Broodlords