There is slabs that you can place 1 of the same kind on top of the other. But you cant place a different kind of slab on another slab. You should be able to combine slabs. I would put a picture i took of what it would look like on here but i dont know how.
http://www.minecraftforum.net/topic/258789-stack-different-types-of-slabs-on-top-of-each-other/page__st__40__hl__+slabs#entry22813624