I make it clear beyond doubt that should I consider it necessary to take command I will do so .
Ok lets say there was an accident and the other pilot said that you were in command and it was your fault. You disagree and say you were not the commander and it goes to court.
The other pilot will tell the court that you said two things before take off before take off
1.should I consider it necessary to take command I will do so
2. and I must agree to this
I think at this stage you would have a hard time arguing that by the authority that you demonstrated before flight you left the other pilot in no doubt that you were indeed in command of the aircraft at all stages of that flight.
We very often fly with two captains and if the other pilot said what you have said I would have no alternative than to recognise that he was the commander of that flight.
At the end of the day its your interpretation against mine and a courts may be entirely different but If I were you I would check my liabiliity insurance, I think you might need it.
Good luck!