It seems to me that it would make sense for groups of new / aspiring writers to team up on writing projects but I have yet to see this happening. Perhaps it is happening behind the scenes I don't know but it seems to be a very solitary, individual thing.
There are numerous advantages in a partnership whereby two people, for example, collaborate on writing a sales letter. Two sets of eyes, two brains tackling the problem and looking for the best ways to create good punchy copy.
The exception might be that sometimes a very particular personal style needs to stay really coherent throughout the page, but who knows when the right two styles might compliment each other (ala Lennon and McCartney, at the risk of making an excessively lofty comparison).