Let's be honest the likes of United and Liverpool have been a very successful club for many years and are a cities divided by a lot less clubs.
The populations of Manchester and Liverpool are tiny compared to London, though. Also that whole north-west area has a huge amount of cities and towns with football teams close together in a way you don't really get anywhere else in England. In my opinion it's time for Manchester and Liverpool to stop pretending and just admit they are suburbs of the same city. But if I go on Red Cafe and RAWK and say that and they call me mad...