This has been debated in the past and will certainly continue as people interpret the regs. If you're logging PIC, do you expect the gentleman officially designated by the company as PIC (name on the release, log book, flight plan etc, flying with you not to log it? He is directly responsible for the safe operation of the a/c, if there is an altitude violation or incident he's taking the fall. When I was Captain on the Lear we sometimes used co-captains due to F/O shortages, it's typed for a PIC and SIC. Occasionally I flew as co-captain; usually in the left seat for my leg, and I always logged it as SIC since I was never designated on the paperwork as PIC for that flight. I guess it's your call, always subject to interpretation