Suggestions on Collaborative DMing

[Cicciograna]

Next month I'm going to run a collaborative Dming effort with a friend of mine: for me it's the first time in such a thing, so I'd like to get some advice from more experienced DMs.
My friend and I haven't yet decided the exact formula for the collaboration, so I can't describe the exact scenario: for example, I do not know which setting we'll play with, or if each of us will control a different character when it's not our turn DMing opposite to the same PC; only thing we know, for now, it will be Pathfinder.

Just for information, I regard myself as a good but not exceptional DM: I tend to railroad a lot, and have an hard time reacting to unexpected actions by PC; the encounters I design tend to be a bit harsh for the PCs, but if I see that thay're in a bad situation I tend to help them; plus, my rewards are rich. Given time I can dish out great adventures, if I have the time to plan accordingly.
The other guy is a much more experienced DM than me: he can write great adventures, with well balanced combats and interesting situations; he's a bit unfair in wealth distribution, though.
For both, it's the first time in such situation.

Suggestions?

[The_Scourge]

I've done this before, my best advice is communication. Make sure you know who's doing what and when. Single DMing is a wee bit simpler because you only have to confer with yourself, with a co-dm you can run into conflicting decisions which can eat up game time.
Other than that, play to your strengths. If you're good at planning and he's better on the fly, run with that. Work as a team and the coDM experience can be amazing.
I'd advise against switching out though, if your styles are too different then it can break game flow. If you do it, have both of you behind the screen for the whole game.

[Endarire]

The_Scourge: I wholly concur.

[Erom]

Having done this several times:

(1) Rotating works fine in my experience - the different styles DO cause some game flow issues, but honestly the effect for me has always been refreshing variability and novelty rather than dissonance.

(2) DO NOT get too attached to any one element of world building. You're going to have to compromise a lot.

(3) Try to stay away from each other's toes - if your partner has drawn up a cool port city, don't start making sweeping changes to the ruling structure, or plop down a dragon in the middle of town. Go subtle when playing in each others back yard.

(4) Relatedly, stay away from world-spanning plot archs, since they step all over everyones' toes. Borrow elements from sandbox campaigns. Consider one-off sessions or short campaigns set in the co-DMed world rather than a single long consistent campaign. Jumping between parties let's you conduct a world tour without ending up getting stuck on a single plotline.

(5) Don't be afraid to ret-con. Often you'll have a cool idea a moment after it is relevant, especially if you're like me and not that fast a thinker. That's ok, if something is cool enough it should be OK to rewind the game a little bit to incorporate it!

[SamsDisciple]

My experience of Co DMing is only with paranoia but it was vital due to all the secret notes passed by players of secret things they were doing. This is actually quite a good strategy to have two DMs behind the screen for a backstabbity campaign. For just swapping backhand forth I would suggest one person plan out the big story line and the other worry about the details with the players