Ah.. JL does this.(though,Not "2" Just one relief-pilot on it.) But as far as I know there's no the "regular" seat. Should be vary... they're just taking a empty seat that I suppose.

Of course, there're exceptions. For example... Mr.Sony, he used to buy-up all of F-seats when he flies with JL. In that case, no one to be allowed to enter the F-section without his permission during the flight, even FAs. So, the relief pilot has to be sleeping/sitting somewhere outside the F-section. I've also seen a relief sitting next to me once on a JFK~NRT flight. He was served F-meal but no booz offered . Apparently, someone took over his F-seat.