I think when you add the two together you get two fantastic players. Right now i think Ibrahimovic is a better footballer than Rooney, this can and should change when Rooney gets back to being a striker and polishes the edges of his game.
FOr me Rooney is a strange case, when he burst into the world of football i thought, "shit, this kid could be the best player around in a few years" but he has changes so much as a player. He's lost that directness, the willingness to run through a man and smash it in, the willingness to trust his left foot and he hasnt improved the weak sides like his heading enough. Now he has improved as a player but the very things that got everyone excited about him are harder to find in wayne.
IBra is the opposite, he's gotten better and better in every way. His left is very strong, he is good (still not amazing) in the air and his temprement has improved 10 fold as he has matured as a player and a person.
That's just my opinion and i'm sure no United fan would swap Rooney for anyone, even for a player like Messi in a lot of cases despite Messi being better. (even hardcore Rooney fans can admit as much)
And by the same token you can bet the fans of the teams Ibrahimovic plays for wouldnt swap him for anyone either.