When you've got two people ganging up on you ur down-b becomes a hundred times more useful, as you can be fairly certain that if you stand still for a second you are going to be hit, unlike in 1v1 in which you often just wind up looking like an idiot.
Like Kylo Winter said, don't use down smash. Ever. To my knowledge, there isn't a single situation in which another move wouldn't work better.
If you are fighting two people, idealy you should keep them at a distance. The great thing about 2v1 is ur opponents tend to get overly aggressive in trying to end the match, as well as overconfident. Keep retreating and striking when you see an opportunity. Of course if your teamate is alive it is important for both players to be aggressive (usualy).
Something that has worked for me a number of times in the past when all hope seemed lost, is fighting them from the edge. If you feel confident with your edgegame, just hang from the edge and alternate between ff and grabbing the edge again (to reduce the amount of time you are vulnerable via the invincibility frames you get from grabbing the edge, & by simply not being within range), and letting go of the edge to air jump, attack with fair, nair, or uair (I reccomend the nair because it can't be sidestep-dodged, and because it lasts a while and you hit them twice, it's easy to DI away from them as you hit their shield [if they managed to raise it in time] to avoid a shield-grab). It's also possible to kill your opponent this way if you DI forward as you attack and hit them with the second swing of the nair while you are behind them. By the time your opponents realize thier folly and back away from the edge it's often too late, and you can return onto the battle field to a more level playing field.
Incidently, this is often a great strategy against low-level players, and in situations where you have a lot of damage and you are likely to die in the very near future (eg. you have 120 percent damage on Corneria and you are fighting a Fox. A single grab to up-A combo will spell your doom.) In such situations your opponent will often be overly-eager to finish you off, and you can take advantage of it with this strategy.
EDIT:
PS- Be careful not to fast fall after an aerial when using this technique. Accidently doing so has killed me many times.