Combination of slabs

I wanted to combine a Stone Slab with a Pine Wooden Slab.

What I expected to happen was...:
I thought i could combine it to a full block.

What actually happened was...:
I only could use one sort of slab.

Steps to Reproduce:
1. I put a stone slab on the top.
2. Then i wanted to put a pine wooden slab under it
3. Nothing happend.

I wish i ould combine thoose blocks.