Co-GMs are useful if you know you can get along with the person. While it limits the pool, we would highly recommend going with someone you've RP'd well with in the past. That way, you have an idea of how you work together, of what each other's quirks are.

As for detailed advice, rather than writing a lot here that has been said before, we'll send you to the guild newsletter we were interviewed about having a Co-GM.
You'll find that [url=http://www.roleplayerguild.com/topics/31820/posts/ooc]here[/url]