When two Captains are scheduled to fly together during regular line operations, the designated commander logs the flight as P1/ PIC/ Commander and the Captain scheduled to fly in the right seat logs the flight as P2/ First Officer.
The designated commander is the Captain who the company has scheduled to be in command for the flight and is the pilot signing the technical log and ultimately responsible for the aircraft and flight.
The roles of PF and PM have nothing to do with who is commander. As mentioned, a Captain needs to be qualified to fly in the right seat but, when they do, they are just doing the job of First Officer.