Holly Lisle has an essay on it:
http://www.hollylisle.com/fm/Articles/wc3-1.html
She's had mixed success with collaborations, FWIW, and if you read elsewhere on her website, she out-and-out says not to do it.
On the other hand, there's teams like Hickman & Weiss or Niven and Pournelle who make collaborations work really really well.
I've done collaborations that have really worked and some that really tanked to the point of pretty much ruining the original friendship.
One of the biggest things I can warn you about is to make certain that you both are totally seeing the same story the same way. While what excites both of you about the story might not be the same, there should be overlap. If this doesn't happen, you'll be writing the story at cross-purposes which--if it gets finished--usually seems very disjointed. If you're both not in sync during the process, there's usually a ton of rewrites because of the lack of joint vision.
One of the pros I've found in my working collaboration is that we have complimentary strengths in our writing. She's really great at worldbuilding & description. I'm really strong in plot. We're both good in different aspects of character. When we're working on a collaboration, we've got the workload arranged so we both work within our strengths and rarely within our weaknesses. However, I have to add that it took us 4 novel attempts and almost 8 years and a LOT of negotiation and discussions to discover how to make this work. It wasn't something we just stepped into.
OTOH, I've had other collaborations where someone wanted to have all the ideas and let me do all the plotting, character building, worldbuilding and writing subject to their approval. And then there were others.... but I don't want this to get too depressing with all my mistakes in the past.
Basically, don't assume this is going to work without extra work, far beyond the writing. It's like any other relationship, even closer than working with an agent or editor because you're sharing the heart and soul of your story from the very start of it. When you joint-create something, you share ownership and there's other legal matters that need be thought about and discussed.
It can be great, it can be hell on earth. Just be aware of what you're getting into from the start and don't be afraid to discuss things with whoever you're thinking about collaborating with.
Best of luck.
