I don't even think the scenario as described above, with a Superman title focusing on new52 Superman in the mainstream DC universe, and an Action Comics title featuring the pre-Flashpoint Superman, returning to a Metropolis in his original universe that presumed him dead the last few years, just as he presumed that that universe was dead, would need to be a trial or a test run.
What's wrong with having two Supermen, each in their own universe? It'd certainly be a way of diversifying the Superman line by differentiating the two main books and giving people separate reasons to buy each of them, instead of them essentially both being the same thing with different individual stories.
I also think, though there might be a friendly rivalry among fans about which Superman is better, the way people argue about Superman and Batman in general, that there would actually be a broad interest in both titles. If their Superman hadn't been eliminated at the start of the new52, and it was just two separate Superman books in two separate universes, the pre-Flashpoint fans might have gone easier on the new52 Superman and come to enjoy him as an alternative take on the Superman mythos. That he was *replacing* their guy is what got some of them really upset. Similarly, now we have the pre-Flashpoint Superman *replacing* the new52 Superman, which is getting some of us newer fans (Although I hesitate to categorize myself that way, I remember watching Lois and Clark on TV in the 90s and stuff, I'm just newer to really following comics) upset that our Superman is gone.
If they'd just told me pre-Flashpoint Superman is taking over Action Comics and new52 Superman gets the Superman title and Justice League books and stuff, I'd probably have subscribed to all of them. As things stand, I may drop all of my subs after Rebirth (Already dropped the two with retro Supermen- Lois and Clark, and The Coming of the Supermen).
DC Comics has twice with their poor treatment of a version of the Superman character created a contentious situation that's alienated some fans. I'll bet both books would sell better if they existed side by side. Every once in a while, there could be a cross-dimensional Super team up. People could buy the "other" Superman's book, whomever the "other" Superman is for them, without feeling resentful that he'd displaced their guy, and just think of it as a bonus added, the way they'd buy a Batman book or a Green Lantern book or whatever.
Oh, and added bonus, those crossovers everyone thinks are too frequent and/or require people to buy two man side books they don't want? There might be less of them if the two Superman exist in two different continuities, leaving more space for each Superman to be developed in their own books with their own stories.
I think editorial probably feels that having two Supermen in two different universes would create confusion, and maybe it did in the 80s when they had a similar scenario pre-Crisis (When what we now talk about as the pre-Flashpoint Superman was the younger guy stealing the original Superman's spotlight
). However, one thing they may not be taking into account on that is that their audience is no longer primarily children. It's adult comic book nerds, and maybe some teenagers. You know what adult comic book nerds are really good at? Keeping track of continuity and alternate versions.
I mean, seriously, the audience DC has and wants more of is exactly the audience most likely to understand two separate Supermen in two separate universes in their own books almost instantaneously and not even need much of an explanation, as long as it's not done in an overtly confusing way. One Superman and one universe per series. There.
They could even encourage the pre-Flashpoint Action Comics to go with retro artistic work to some degree (Not necessarily like Golden Age stuff, just 80s and 90s and 00s stuff- post-crisis), and the new52 Superman title to go with cutting edge stuff. The whole look and feel of each series could match the era each Superman started in. Differentiate the costumes more. You could have new52 Clark really go after Diana. You could put pre-Flashpoint Kent back in the fedora and trunks and reporting for a paper newspaper (I don't know if you'd actually change the time period, you might just say it's happening "currently" and since it's another universe, it's a mix of classic stuff and modern stuff, like the TV show Gotham, which features classic cars on the road, flip phones instead of smart phones, and yet has some very modern stuff as well, where it's really in it's own universe).
I'm sure there'd be a snarky editorial or two about audience confusion from clipbait websites and media that doesn't really get comic books or comic book fans, or people who do get comic books and comic book fans, but are just trying to stir the pot, saying the whole situation that would exist with two Supermen in two different comics and two different universes was needlessly confusing and a sign of editorial dissarary, but that wouldn't make it *true*, and I don't think much of the core fan base and the people you realistically might reach would share that impression.
The truth is, there are three markets: a market for everything Superman, a market for a retro Superman, and a market for a modern edgier Superman. You own a comic book publishing house that used to publish almost 100 monthly titles, down to 52 in 2011, and 30-40 post-Rebirth (Although with some shipping twice a month, it's really going to be fairly close to 52 again, I'd think, if we count issues per month). Superman is probably your first or second best known character. It's not a grave hardship to do one book to make one audience happy and a second book to make another audience happy- with that third audience that likes both cross-buying both titles (Also, people from each camp becoming part of the third camp just because, hey, their Superman is no longer threatened, so why not give the other one a try?). Each book would probably sell better individually than giving both of the major Superman titles to one version of Superman.
I mean, when there were four Supermen after the Death of Superman storyline from in the 90s, people didn't like that, but it was because it was essentially four versions of Superman that nobody wanted, and they were all in the same universe with questionable claims on being that universe's Superman. Same with Superman Red and Superman Blue. This would be different, because it's two Supermen who each have large constituencies, each in their own universe, where they were Clark Kent and are definitely their universe's Superman.