By the time you have to worry about the fence and the hedge intermingling - you'll probably be having to rip them both out again and redoing it anyway.
Personally leave about 1.5 m either side - we have fences on top and next to original hedges - the major downer with the ones on the side is that sheep get on top of the hedge and jump the fences. I am trying like you to do either side but its expensive and time consuming as you probably know.
Dunno if its any help - but as I can only afford to do one side at the moment I have put them on the side facing the previaling wind so that the hedge growth goes towards the blank side and not the new fence - I can then trim these easily.
Baz