Oh boy.
I just wrapped up a game run by 2 GMs (not D&D, though), and the experience was satisfying because, as the previous poster stated, our roles were clear. We respected each other's agreed-upon GM space, but we also collaborated on game-affecting issues, particularly when one GM's sphere of influence affected the other's.
Since D&D is a different system from what I used, there are added complications that need to be ironed out before the first character is rolled up. As the saying goes, an ounce of prevention is worth a pound of cure. Some tips I can think of right off the bat are:
1. Determine the structure of the game. How do DMs divide responsibilities? Are these responsibilities focused on rules and tactics or on setting and story, or both? What spheres of influence does each DM have? How will the players be affected? Do you stay with the traditional party structure, or do you do something different?
2. Make sure you are on the same page concerning rules. While no two DMs will do things exactly the same way, you need to have a clear picture of the rules you'll be using to play the game, including house rules.
3. Use the two DM structure to your advantage. With a group as large as yours, this would probably be essential. Since with two DMs you won't need to stick together as a party, consider the possibilities of having one DM be available to run things with characters who go off by themselves or who do things in secret.