Steve Almond and Julianna Baggott wrote a book together. Stephanie Merchant at Bookslut spoke to them about the process:

The upside of the collaboration: this is a competitive book. The early back-and-forth of chapters was marked by the good energy of one-up-manship. The downside is that when things fell apart, as things do at some point (or many) while writing a novel, we didn't get to bully through these bouts alone. Almost strangers, we got to know each other's brooding artistic temperaments quickly. Some writers can really pull off a good bout of brood, but we didn't wear these temperaments with grace or charm -- you know, like matching black ascots or something. No.

More here.