we use two drummers in much of our music - it's always a tricky situation as it's easy for both overplay - thus we spend a lot of time experimenting with different combinations and contexts where having 8 limbs hitting things instead of 4 makes sense. density is a big talking point at rehearsals. it's easy for things to devolve into a spaceless rant of constant 16th notes.
it is very interesting to watch two drummers play together in a improvisatory context (and I don't mean guitar center drum battle 2010 or other wankery.)
here is an video of two drummers nasheet waits and eric mcpherson performing in a trio with abraham burton. I think it's a good example of how organic two drummers can play. Sure, it sounds dense at times, but density, just like volume, is something a drummer can control, and two drummers can do interesting things with that. rhythms can clash in ways that one drummer could not achieve.
(you can view all 6 parts of the full set on youtube where they play in a bunch of different contexts within the trio, this is just one segment).