Depends on how strong a signal your neighbour is receiving and how may times the feed is split to feed his equipment. Other than that it's just a question of splitting the feed with a splitter(see
here). Length of co-ax may be also be an issue, as the longer it is, the weaker the signal will be at your end.
As to written agreements, it's a free to air signal, so nothing illegal in wanting to do what your doing, but i'd put something in writing that shows you've got the providers consent anyway. This should also specify time span, and that the provider has no responsibility for the provision of the signal, and that you don't expect it to be there.