Neither have I. I was planning on doing it once, but my friend backed out because of college. I requested a friend write a fic for me, and helped her plan it, but I've never really collaborated with anyone.
Whispering Memories - FanFiction.Net
^Note that there aren't any Degrassi fics yet, but hopefully you're either familiar with one of the fandoms I've written for, or you're only looking for the style. If you want to read what's really my best, read "Toy".