Or, if you wish, have them owned by the same benefactor.
At a number of points in time we could have Disney buy them both...Your challenge is, at any point after their OTL foundings, to have DC and Marvel merge as companies. Or, if you wish, have them owned by the same benefactor.
Going off of the above poster, Jack Kirby (also known as the King of Comics) worked with Stan Lee for years in Marvel and also worked in DC comics. Maybe have better collaboration between the two men creating their characters, which leads to a better relationship between Marvel and DC? After all, Kirby did move between Marvel and DC for a good part of his career. Eventually, the two companies merge into a single corporation.
I don't know how it could happen but having Stan run the merger would be truly ground breaking. Also there is several crossovers already with out the said merger. I'm interested in how the crossover would take place with this merger. Could we also see a longer lasting Amalgamation series.
Both editor have a multiverse so they can be depicted simple as another alternate universe earth......Maybe a early crisis of infinite earths type cross-over, entsablishing the respective role or the two earths, becouse frankly Gotham and Metropolis feel ill suited for the Marvel universe