HOLY CASTING! It's almost enough to leave you Speechless. MGM's current romantic comedy features not one, but two ex-superheroes: Michael Keaton (formerly Batman) and Christopher Reeve (formerly Superman). On the set, the dynamic duo agreed their men-in-tights days are over. ''I was the custodian (of that part) in the late '70s, early '80s,'' says Reeve. ''If they do it again (producer Jon Peters
may revive the big-screen franchise), there should be a new custodian for this generation.'' Keaton, who relinquished his role to Val Kilmer in the upcoming Batman Forever, echoes the sentiment. ''I pretty much had done what I wanted to do with the character in the first two Batmans,'' he says. ''I liked being a part of it, (but) there are a lot of other things I want to do in front of the camera and privately.'' And will Keaton see the new Batman, due this summer? ''I'm so far behind with my movies it'll be 1998 before I see (it).''