How do you draw in an opposite color bishops endgame with two pawns down? Is it possible to draw in such an endgame? Are there exceptions? If so, what are they?
Yes, if the pawns can be blockaded it's often an easy draw. One example:

[fen "8/4k3/4P3/3P4/2B1K3/b7/8/8 w KQkq - 0 1"]


As long as the black bishop controls d6, the pawns aren't going anywhere. 

But two pawns are two pawns. If they are farther apart and far progressed often one pawn wins the bishop and the other pawn wins the game, or one pawn sacrifices itself so the other one cannot be blocked by the bishop anymore. But this depends heavily on the specifics of the position.