I am currently in the process of running an Eberron campiagn, with another DM.
It happened on a fluke, when Eberron was first released, we did a demo for it. The other Dm was impressed with it *the session*, and ask me to run side by side with it. The demo later grew as a campiagn down the road.
So far, it works well, cause we both understand the importance of one rule. The players' enjoyment comes first. And second, the zaniness that come from the two of us, works well, and we can bounce an improv situation of each other that quickly and work into the game.
And if by chance, if one of us is out for some reason, the game will not be held up or cancelled on short notice, cause there is a part missing.
And mostly, if there is no ego involved, the sessions can be run with the miminal fuss, and mostly there must be an understanding. That each DM there, is not there, to showboat who is greater in storytelling or whatever else. If there is general path being followed, and whatever steps are being done to get there, is decent and proper, as long both DMs' are aware of it, in general.
Communication is the very important number one factor, in having a great and successful dual-Dming stint. If there is no talking between gamemasters, before the game, during the game and after.
Don't bother joining up.
And oh, one more thing, Dual-Dming is usually done, when your gaming party is in the double digits...doing for less than that catergory number...one will suffice.
Or, as someone else noted, a party split will have the use for a dual-Dming, only if the situation is truly warranted.
But that depends upon many factors...
Almost forgot, we be hitting one year mark soon in doing this.