We had one primary DM, but several of us served as "alternate DM", maybe a quarter to a third of the time. It was a fairly detailed "shared world"; the primary DM had one city (City-State of the Invincible Overlord) that was "his", but we all DMed in the same world.
We all had multiple PCs, of varying levels, though we always played one PC per player at a time. Which PCs we'd play would depend on what adventure was being run -- we did a lot of TSR and Judges Guild modules, and the "campaign" was only a campaign in the loosest sense (there was little or no metaplot).
Eric (the primary DM) would say, "I'm going to run White Plume Mountain, it says it's for levels X-Y, you guys figure out which of your PCs you're going to play." Then, we'd haggle over who'd play which class (in hopes of some party balance), and play that module (which might take multiple sessions). Once we finished that module, Eric (or another DM) would say, "Next, I'm going to run (module Z)", and we'd figure out a party all over again.
On occasion, we'd play a longer series of adventures (like the Giants / Underdark, or the Slavers) which would see most of us locked into a particular PC for a while, but that was the exception to the rule.
Back then, I don't think it ever occurred to us that each DM would have his/her own campaign, or dedicated PCs for particular DMs.