RpNation

I've tried it a handful of times I believe, though the only concrete example that comes to mind is a my hero academia RP I've done quite a while back. It was actually one of the longest lasting group RPs I've done and I remember I originally got co-GMs partially because I was coming up on summer and I'm usually dragged out for family trips and partially because I had a very active (in a good and bad sense alike) style of GMing which was starting to be overwhelmed by the sheer number of players/character applications.

I've also been a co-GM myself once also in a my hero RP (my in-depth look into character sheets was appreciated by the GM and they invited me to help out with the CS review part of the process) and once in a fantasy RP where each co-GM was to run their own storyline for the players, with some key points to follow form the GM.

I see. Would you be open to cogming something else? On the sister side. 100 or so posts long?

It seems you have something specific in mind. I can't promise anything since I tend to be picky about the kinds of roleplays I join and it's definitely harder to convince me to join something that already started, but I'm not entirely closed off to the idea. So I can hear you out, if nothing else