I didn't know that was possible. Sounds like a recipe for conflict to me. Its fine when you get on with your neighbour but people move on.
How do the deeds word it? There must be some allocation of responsibility surely? I have a very old house and of course there are no 'T' marks (what a good idea) but the plans are clearly marked to show ownership and responsibility for the boundaries. None of them mine, I'm pleased to say, as there are quite a lot of them.
Peter Scott