There's no reason why it can't be the same earth, the same universe where the characters don't interact with each other or mention other events.
Back in the 70s and 80s, Marvel and DC published cross-overs. There was no multiverse hopping, Metropolis and Gotham co-existed with New York. Just that Superman hadn't got around to meeting Spider-Man. Simple and nothing to explain from professional writers who drifted into comic-books. From the late 80s onwards, as more and more fans become writers, they bough bough their fan think and fan fiction to the table. Stories that would explain stuff that really didn't need explaining. That's what the Marvel and DC universe is like these days.