Minecraft wiki entry for Slab, associated text: Placing a slab on top of a block or on the side of a block in the lower half of the side surface creates a bottom slab
Placing a slab on the underside of a block or on the top half of the side surface creates a top slab
Placing a top and bottom slab of the same type in the same block creates a double slab block
It is impossible to place two different kinds of slabs in the same block.