Co-Authoring?

After reading through the "Two Cheers for Peer Response" article again, I became somewhat intrigued with the concept of co-authoring.  This "in-class, co-authored writing assignment" entails students being given forty minutes to compose a paragraph together.  I'm curious if any of you teachers have done an assignment like this in your classroom.  Was it effective?  Ineffective?  Also curious as to a possible prompt for this kind of assignment.  I think constructing an argument would be difficult if two students with opposing views were paired together.  Do you let students pick their own partners? 

I'd like to try this out in a workshop but definitely wouldn't mind some more direction before I dive in...