There are a lot of strategies that can get you a quick win if the enemy fails to counter it, usually those that rely on a single unit type produced very early in high quantities. The best known examples are double 6/7/8 pools and double mass-marines from 3rax with two reactors and one tech lab for stim.
They work, they are strong, but if they don't kill both enemies fast then you are at a significant disadvantage. I do not recommend using them though.
There are a lot of combinations that work very well, but most depend on your opponents and their strategies and of course on your races. For example against two protoss going two-gate mass-zealot a Sentry/Marine force works quite well (sentries forcefield the zealots away from the marines so the marines can make use of their insane DPS) and guardian shield will also help against double mass-marine.
I do not recommend teching too fast unless the enemy turtles (for example if they have multiple cannons at the ramps). My usual 14hatch/13pool into mass-zergling/baneling strategy as Zerg is the same as my 1v1 Strategy and unless there is a 6-pool it works quite well, too.
If everything fails, try to die slowly, as Day9 says