At our Firm we give every Captain a right seat check on every recurrent (2 x p. yr) Precisely because the opportunity exists that two Captains may operate a flight for various reasons.
My personal experience is that there is a little more fumbling but the higher combined experience level of two qualified Captains more then make up for that. Additionally, I find that the handling Pilot will be very much sticking to the SOP's because s/he knows that a Captain in the right seat may not be as reluctant to speak out against possible deviations then an F/O would be.
Further, I welcome the opportunity to see how other Captains run their ship, it can be a learning experience either way!
When we were operating 727's, I prided myself, as did many of my fellow Pilots, to be as sharp on the panel as any F/E, eventhough I never held that position.