Planning a campaign in which I'm hoping to be a part time player, with a Co-DM?

[MonkeySage]

I'm not sure how this is going to work- I just know that I am always stuck being the DM, and as much as I enjoy DMing, I would like to be a player. So I decided to begin work on a 3.5 campaign which I'll be running as the DM most of the time, but actually spoke to a friend about him possibly Co-DMing so I can also be a player. When he's DMing, I'm not- I'm playing. It'll be my setting, which I've been putting a lot of work into for the past couple weeks- worldbuilding and stories, stuff like that. However, my goal is not to give myself an unfair advantage over the other players whenever he takes over as a DM. When I'm not a player, my PC character will be on downtime, which I'll be working with the Co-DM on.
I want to handle this as fairly and cleanly as possible, and I'm thinking when we know Co-DM will be taking over, I should plan the transitions- so he has his main story and I have mine within the same campaign.
But, how should I do this? How will I be able to avoid potential problems.

[Batcathat]

If you're the one who made the setting, it's probably important to make it very clear how much freedom he has with it. Can he make up whatever he wants that's not already written down? Will you have any authority to "correct" him when he's GM:ing if you don't like his interpretation of something? And so on and so forth. Whatever you agree on can probably work but I suspect it's important that the "rules" are clear to both of you from the start.