Farside, you must be able to leave your seat - even if only on a 30 minute sector - if the physiological need arises, and that could mean a quick 3 minute slash, or a 5 minute stand, to prevent thomboses forming.That is inflight relief..think about it in a 2-crew scenario...one Capt & one F/O.
If, because of duty time, or the effects of Circadian rythm, the crew requires rest, THEN you will NEED at least 2 Captains, in a one on/one off role.
Personally, on the "heavy crews", I believe there should be 2 Captains' names shown on the ATC lodged flight plan. That would stop any shenanigans on the companies' part!