I started gaming in late 1980. There were four of us in the core group, and each one of us regularly acted as DM. None of us had a distinct campaign world. Instead, we all simply ran adventures in what might be called "D&D land". There was little to no continuity between one adventure and the next. Sometimes as players we'd use player characters from previous games, but at least just as often we'd simply roll new characters. Whoever the DM was would say, "OK, bring characters of levels X to Y."
So in a way we all co-DMed an indistinct, generic D&D world (with no place names that lasted for more than a single session).