>>Actually, you'd probably be surprised by the number of books written under a single pen name that are actually authored by duos.
>
>My favourite was Bustos Domeq <g> http://en.wikipedia.org/wiki/H._Bustos_Domecq

In the mystery area, right off the top of my head, there are P.J. Tracy (a mother/daughter team) and Charles Todd (a mother/son team). Hard to tell when you read them that they aren't written by a single person.