In Warrington its very common. Every supporter i know who follows Wire also follows United, Liverpool, Everton or City. St Helens is the same. The pubs when these teams play (esp United vs Liverpool) can get interesting. I can be in there with a mate and we'll be in direct competition, having a go at each other and telling each other how there dirty Scouse this, or Manc scum that. A few hours later we are stood next to each other on the terrace bagging Yorkshire Sheep erm Lovers or Inbred Pie eaters (even though the few of them i know all support United and we sometimes watch games together!!).
Great fun.
In Wigan its a bit different as the codes are competing, and the Football (soccer) only recently got good. Most Wigan rugby fans support (Mainly) United or Liverpool also creating a rivalry there.
i imagine Yorkshire is similar but not as extreme, Soccer is much bigger in Lancashire than Yorkshire. Thats a guess though as i don't go that way often.
Its very rare a Soccer fan in say Manchester or Liverpool will follow a rugby team though.
Most people in the area dislike Union. Mainly 'cause of History. Also Sale is a bit poncy...