This has been done to death. Easy, If there are are two pilots on board both current and qualified on type and can safely reach all controls, they CAN decide who is P1 at ANY time they choose. And must log each portion of the flight they did.
Dont make it complicated.
No, they can not legally, at least to my knowledge - which was the question wether I am on track. There are no split responsibilities in flight, for good reason. And as reminder, we are talking PIC, not "you have control".