It is clearly legal - insurance permitting - for two people to go up in a single crew aircraft, and swap over the PIC duty in any way they agree between themselves, and log it accordingly.
It would not make practical sense to enforce a prior "PIC" designation which must last for the entire leg, takeoff to landing.
If anybody tries to enforce a law which says differently, it isn't going to work, and whatever people do now they will continue to do.
One difference, I guess, will be that if A and B share the flight in a plane owned by A, but B ends up having (to comply with some law) to log it, then A will want to charge B some money because B's logbook has got the benefit. Unless of course A is not interested in logging time. In any case, they can straighten up the expense sharing after the flight
Unless there is a prang, with only one of the two surviving, but that situation can occur anyway, today.