Windows Across Floors

[MarkA]

So I've been struggling to add windows across floors. I have managed adding windows across rooms on the same floor fine by merging the rooms, adding the window then re-dividing the room. However when I apply this approach to adding them across a floor boundary this does not work. I can merge the rooms, add the window, but fail to re-divide the rooms.

any advice? what tool should I be using the split the rooms again?