I've had two experiences with co-dms, and both involved crossovers between our respective campaigns.
In the first one, one of my old friends took over my campaign for a handful of games as the party went into his world and participated in a battle against his ultimate mortal villain. It wasn't exactly co-dming because, when I 'passed the hat', he took over the game until we were done on his plane/campaign world. Then he passed the hat back. It was really cool, really fun and really smooth.
The other time was not too long ago. A friend of mine, who ran an epic dnd group of his own, and I schemed together to make our respective parties fight and then join forces to kill Asmodeus. It was very 'JLA vs. Avengers' in style; it's in my epic story hour, and involves all kinds of crazy manipulation and stuff. We took months to set it up, and arranged it so that each party thought the other group was terribly evil. None of the players knew what was going on in advance; it was a super-secret special double bag collector's item surprise kind of game, if you know what I mean.
When we ran the game, we ran it together, but it tended to be me doing more of the running and him doing more running npcs and stuff. It was a ton of fun, but I think I tended to hog the spotlight a little too much.
I would have problems with co-dms as a regular thing in an ongoing campaign; each dm would inevitably be missing some information that might prove vital to verisimilitude in the future.