I am assuming you mean two drum kits. I think having two percussionists playing different collections of instruments, even if one is a drum kit player, speaks for itself and needs no defence. read more
For a band playing out, the two can play different parts/setups to which they are suited. Some bands I've known have had two drummers partly so that when a schedule conflict arises they can still play a show with just one, and when both show up they have expanded parts. For practice is where it shines in my own experience. read more