Bale's Batman and Routh's Superman fit together very very well. They're hitting that same age range, the movies themselves are tonally similar enough that it's not...jarring to imagine them interacting, and then they would totally get their respective bitches on while fighting crime together.

And you *know* that they had a snarktastic little convo during Superman's pitstop in Gotham.

Supes: "Hey. So...I'm back."
Bats: "..."
Supes: "Did you miss me?"
Bats: "No."
Supes: *big smile* "You totally missed me."
Bats: "I hate you."
Supes: "Tell Alfred I said hi."
Bats: *throws a batarang at Superman's head*
Supes: "You know, a hug says hello just as well as a concussion."