I wouldn't be against the idea of an alternate reality Superman getting some consistent panel time. I mean, I've been waiting for the Justice Incarnate title with President Superman for years now!
But splitting things and making one of them "post-Crisis" and the other "pre-Crisis" feels like giving up to me, somehow. We *can* have a successful, fully integrated Superman who embodies the best of all eras, who all fans (or most of us anyway) can get behind and enjoy.....it just takes some interest and effort (and DC generally lacks both when it comes to Clark).
I mean, yeah by all means bring back Kal-L. He's the best! But I dunno if the answer is "A Superman for both groups of fans." And after the treatment Kal-L got from Johns......I just wanna see that guy get the peace and rest he deserves. I hold the "original" Superman in very high regard and don't trust DC to not take a crap on him (again).
But if they did go this route I wouldn't mind seeing Morrison's t-shirt Superman in the pre-Crisis role. I think at this point there's more differences between Nuperman and current Superman than there are between current Superman and Kal-L. Current Clark has got to be in his 40's or so, is grown and mature, experienced, married, is raising a kid.....he hits a lot of the same notes that Kal-L used to. Nuperman, with all his rash youth and Golden Age spirit (not to mention the costume) stands out comparatively, and if you're going to have two Supermen running around, one embodying pre-Crisis and the other post-, I think you'll get more differences and mileage out of Rebirth-current Superman and Nuperman than current Superman and Kal-L.