Originally Posted by
PotatoChip
Instead of just thinking, why don't you actually look up the regulation???
FAR 61.51
That's the answer. Period.
14 CFR 61.51(e)(iii). When the pilot, except for a holder of a sport or recreational pilot certificate, acts as pilot in command of an aircraft for which more than one pilot is required under the type certificate of the aircraft or the regulations under which the flight is conducted.
There may be two captains assigned to a trip. The one designated PIC logs all the time.
61.51(f)(1) pertains to second in command, in most cases a first officer. 61.55 states SIC qualifications. They can log the time they occupy the seat.
That said, savvy logbook reviewers may or may not take a dim view of a SIC from logging the entire time for a long haul flight.