For me the question isn't really appropriate...
In answer to the question should United and City fans come and watch Latics, I'd say "No", why should they, they support a different team. If United or City were at home and someone offered me a ticket for £20 i'd say no chance.
If the question was "How would you feel if you saw a United or City fan at Latics?" I'd say the more the merrier. I might be concerned they were insane for spending £20 for watching a team they don't support, but providing they were paying for a ticket i'm all for it.