Good points Captain Stable, multi-crew flying is not simple legalities or policies. Even the traditionalists would have to agree from the history of accidents that the individual human element and human interaction (or lack of it) dominate causal factors.
Those people still in the industry must continue to strive for professionalism; it is of little value in being a professional Captain / Commander if you are not going to be a professional co-pilot (as a Captain) when in the right seat. It is both the recognition of the change of role and the additional joint professionalism that a two-captain crew requires in order to defend against the hazards identified in this thread.