Max Angle, maybe a slip of the typo on your part but "The guy who signs the tech. log is the captain..." - I was under the impression that it was the operator who specified who the aicraft commander was - ie whoever's name is in the "Commander" column on sign in is precisely that - the Commander.
Cabin crew can unwittingly get confused when two four ringers are flying together (ask any Training Captain) but it should be made clear to them in training that there is only ONE commander on the a/c.
Captains flying together tends to be less than ideal and Training Captains flying together can be even less ideal!