Changing cat litter can be iffy. Did you switch by mixing the old litter with the new, slowing increasing the new stuff?
I never used wood litter, so I'm afraid that I can't give you any specifics, but I think that your kitties just liked the previous litter better, or they like the idea of using one box for pee and the other one for poop.
I use Yesterday's News, made from the recycled paper. It is very low dust and unscented, but still controlling odor.
Sorry that I can't be of more help!