I'm sort of the secondary DM of the group. However, out of the the 5 people in our group, 4 of us DM from time to time and have our own homebrew worlds and preferred settings. Back before people started moving away and we had 10 people in the group, 8 out of the 10 of us were DMs at one time or another and we each had our own homebrew settings.
Essentially, our gaming groups have always been made up almost entirely of DMs, who would occasionally trade off on which game we'd play every some many sessions, usually after the completion of a story arch or campaign plot point was resolved, or a dungeon was cleared, or the like. The current main DM of my group, however, has always been the "main" DM of all of us, and we played his game more than all of the others combined.
But having a group composed almost exclusively of other DMs is I guess the big reason why I've never encountered the likes of the horror stories about people abusing rules or creating havoc with the game and so on. We all now what it's like to DM and don't want to give him a hard time, we want the game to be fun for the DM too, and we don't want to exploit possible loopholes in the system to make a character that will take spotlight time away from the other players. I think that the best groups are always composed of people who both play and DM, because when you know what it's like on both sides of the screen you're far less likely to give the DM grief, or to try to abuse the rules for your own benefit, because DMing brings "gaming maturity" in a sense.