Not that odd; I used to play with a group where three of us were DMs; One guy DMed 4e on Wednesday night games, I DMed Pathfinder Friday nights, and the third guy DMed a 4e LotR game on Monday nights (although I didn't personally play in this one because he was basing it on the Silmarillion and I had no idea who any of the NPCs were; I subbed for another player on the last session and yep, no idea who any of these characters were).

Most of the friends that I have that are into DnD are in at least two games. Some people like doing it more than one night a week and it's hard to find two days out of every week that everybody in the group is cool spending on it.