Herman: Counter-example...
KjeldPetersen: I would call this example "fetus in fetu", and my theory does not cover this yet. But maybe when I develop my theory more I can include this also. Also seki is not covered. So thank you for your point. I changed the headline to exlude "fetus in fetu".
For each connection two squares shares a corner stone can bee omitted. But it must be omitted so that the connections are not joint over the corners. In your example there are for connections between the two squares so four corners can be omitted.