You can even go
three times a week if you manage your volume wisely. That is a
big if, by the way.
I do what essentially are full-body workouts M, W and F. The key is not overworking any one group with needless extra movements and (as has been said) going to failure. I stick with one major movement per muscle group (legs/back/chest/shoulders-traps) and toss in some bis/tris work here and there.
For exmple, I will do 3-4 sets of
flat bench on M, 3-4 sets of close-
grip bench W (primarily tris, anyway) and 3-4 sets incline/decline bench F. With back I might do rows M, Chins W and pulldowns F. For legs/lower back I'll do deadlifts M, squats W and straight-legged deads F. You get the idea.