Isn't interviewing in pairs fairly common? When being interviewed, there's always been two people across from me for the technical rounds; anything 1-on-1 is limited to initial phone screens and final rounds with the bigwigs. When conducting interviews, I've always been one of two, with the exception of one time when no one else was available to join me, which I'd prefer not to repeat.

I would go so far as to say a random passive observer is still a great idea in addition to two active interviewers.