What should have happened was that when I stand on a slab which is on the lower side of a block i could place a slab on the FULL block two blocks above so there is 2 blocks space in-between.

When i stand on a bottom slab i can not place a upperslab above my head onto a FULL block (so there is two meters distance in-between). I can although place a slab there if i place it onto a upper slab.

Stand on a lower half slab and try to place a upper slab on the full block on top of you, e.g. a upper slab onto a stone block. If you actually place the slab ontop of you onto an other slab it will work.