Starcraft 2v2 Strategies usually involve either an early wall with fixed defenses, or an early army to either attack, or defend-then-counterattack. If you're the one on the attack, and you notice that your opponent has built an early army, you are far better off doing a bit of scouting and then retreating rather than going head-to-head with his army, when he has the defender's advantage.
A common mistake when early-attacking is to bring your combined army into one opponents' base, but then get pincered into the base by his ally's army. That army, plus any reinforcements the first opponent may field, might be enough to hold you off and have enough for a counterattack. I've lost many games doing this.
The other important factor, which I've mentioned in another blog post about 2v2 macro, is to continue to macro when attacking, such that if you DO lose your whole army on the attack, then you have enough defending units to fend off the counterattack.