We added block stacking to blocks that give island value.
There are a few known, not critical bugs we are aware off and these will get patched very soon!
To stack blocks you place 1 block you want to stack. Then you hold shift while you place another one on the one you placed. You will see the block becoming x2. Be carefull with this! Its in development. When you are stacking blocks, make sure you don't stack too fast.