Pros and Cons of Co-participation in Usability Studies
http://www.stcsig.org/usability/newsletter/9804-coparticipation.html

Wilson, Chauncey E.
Usability Interface
1998

Abstract:

Co-participation (also known as co-discovery) is a usability evaluation technique where two participants are paired in a usability test and work collaboratively on tasks. They are often asked to think aloud while working together. I've used co-participation for both hardware and software tests and have also used the more traditional one-person-at-a-time technique.