Singers do them all the time. Cash and Dylan. Paul McCartney and Michael Jackson. Mr. Fox and Mrs. Polar Bear. I'm always surprised that writers don't follow suit more often.
Steve Almond and Julianna Baggott wrote Which Brings Me to You together. But off hand, I can't think of any other examples. What would you pay to see Don DeLillo and Thomas Pynchon join forces?
Admit2 is a journal dedicated to collaborative writing. They're reading submissions until September 1st. If you're stuck in a rut style wise, then working on something for admit2 could be a big boost. Like Almond and Baggott, you could do an epistolary style, each partner writing one half of the letters, or perhaps each of you can write a line and trade off or whatever method you conjure up.
So find a friend and get writing.
photo by http://www.flickr.com/photos/tommy_toast/
