Folks,
When two captains fly together, the most difficult task befalls the person designated as the FO.
It is he/she who must pack up the ego and the "must be in charge" tendencies and devote all his/her skills to being the best FO with whom the Captain could hope to be rostered. That person must ensure that he/she does everything to ensure that there is no confusion on anyone's part as to who the operator has designated as PIC.
The next most difficult task is for the designated PIC to understand that the person acting as FO is in an unfamiliar environment doing things that he/she normally only supervises and therefore a double dose of patience and vigilance is required.
Instead of banning the practice, I would advocate making it mandatory - it is great for self-awareness, humility and self-discipline. Don't avoid the difficulties - do it to ensure that you do not become the problem!!!!
Stay Alive,