I too live in a shared freehold. I have jad no problems but I think that's because the lady upstairs and I agreed that she would take care of her flat and I would take care of mine. Some people agree to have a monthly "kitty" where you both pay in an agreed amount, should any joint repairs occur. If you had a flooded kitchen would he have to help pay? What if there was a major problem with the roof? Who would pay?
I don't have a kitty, but I do pay her �16 buildings insurance a month. She pays the other half. (She showed me all the paperwork etc).
I had to pay to have railings put up outside my front window. she paid for a large tile to be removed from her guttering by the roof. We just take care of our own bit, basically.
This is fine if your neighbour is reasonable. If you think he may not be, it might be an idea to go to a conveyancer or solicitor and agree the rules with him, and sign them properly.
You will also both own each other's leashold, so you are each other's landlord! Check the lease and see what it says. Mine says all sorts of weird things about painting the outside every 6 months etc, but we agreed to ignore it, and just do things as and when they are needed.
You would also need his permission if you wanted to let the flat out. If your tennants trashed it, it would affect HIS property price.