A friend and I have decided to co-run a third edition campaign in which we take turns, adventure to adventure , in the GM chair. He is first up and will be kicking things off with Witch's Song. Actually, this will be the re-kindling of a campaign that he started with two other players, both of whom are returning for the continuation. The first time around, two sessions were played. I will join with a character that I have not yet made. The first adventure was a home-brew of his design.
A bit of back ground on us. He has been playing since 2nd edition and has GM experience most recently with Dark Heresy and Rogue Trader. I have been playing since first edition, mostly as a GM. Both of us have a vast world knowledge and neither of us have a great amount of experience with third edition.
After the group completes WS, it is my turn up to bat. At that point, my co-GM will either make a new character and join as a PC, with my character going to NPC status; or, he will take over my character, making him "our" character. We have not yet worked that part out, nor have I yet decided, but do believe I will run Eye for an Eye first.
The swapping will continue with the adventures that we will run being (* = adventure run by me), numbers in [brackets] = number of four hour sessions I anticipate each adventure to take. This is not necessarily the order we will run with:
Witch's Song 
Horror at Hugenhal 
Crimson Rain 
The Gathering Storm 
Eye for an Eye* 
The Edge of Night* 
The Mirror of Desire* 
The Winds of Change* 
That may or may not be the end; however, now having at least done a brief read of the adventures I am to run (and avoiding those I am playing in), I am not sure that our initial idea of trading chairs adventure to adventure is the best idea. Eye, Edge and Mirror make a nice trilogy as all three take place in Ubersreik,and share a cast of NPC's. With that being the case, of the adventures he will be running, do any of them make a nice trilogy for him, thematically or geographically? I realize that the Gathering Storm is much longer than the rest, but that doesn't concern me. Are my guesses on number of sessions needed per adventure accurate?