Honestly, you probably won't be going through litter any faster, although you'll have to start out with more just to fill both of them. I have three now, with the two kittehs and I figure it won't be too different than having two. I'd rather have the babies spread out their business anyway, because it means that I'm less likely to need to clean the boxes out every single day.
I haven't settled on a litter yet, but right now I've got Kitten Attract, just to make sure that Beren goes where he's supposed to. But I like Swheat flushable litter. Not sure how long I'll actually use it (I'm moving to a place with iffy plumbing at the end of the year) but I like that and the kittehs don't mind it.
Wyn is a bad, bad kitteh. Good thing her person is a bad, bad girl... They get along great!