This is a damn good question. I never could get a clear answer on it but here is what I did. We had 2 Capts and 1 F/O as an augmented crew. I only logged the time I was in the seat and then in the comments section I put "Augmented Crew" and the total time.
To make it even more confusing both Capts names were on the flight plan, so how do ya log PIC on that? I still logged the time I was in the seat as PIC.