I ran a very short-term campaign for 2 players. I let them each have two PCs. This only works for experienced players, in my opinion. New players have enough trouble juggling one character's abilities and attacks, unless they're very quick on the uptake.
I know of two other people in my group who are involved in campaigns with only 2 or 3 players. In one there is a regular NPC who helps out the player-characters, and the GM is planning to let the players run two characters each so she can take a break from running the NPC. In the other game, which has 3 players, the characters have several NPC droids giving them a hand (it's a Star Wars campaign).