As suggested above, maybe Bruce could be Batman on a more global or cosmic scale with the JL while Dick remains the Batman of Gotham City.
I like this idea, though I think that Dick would be more likely to be "global Batman" or "JLA-Batman," as his former duties (and memberships in the Titans and whatnot) have shown him taking on a more globe-spanning role, while Bruce has maintained for years that things like the League take him away from the time he'd rather be spending in Gotham.
Having two Batmen could perpetuate the myth of Batman as a supernatural creature, if he's showing up in two places at once.
Yeah, I really like this idea....
(That's why they won't do it...)