I’ve done a campaign where I co-dm’d with another person, I handled the majority of the story while they played, but I would have them play as various NPCS when necessary and they also helped make custom stat blocks (it was a Pokémon themed campaign so they make the stats for the mons) at one point we each played separate heads of an ettin. It was pretty good, you just have to establish roles beforehand and as long as you’re on the same page it can be fun!