Some operations do use two PICs. Not really a nonsense term in all cases.

Some flight departments consist of exactly two pilots. If there is only one true PIC, then the whole operation would shut down if that one pilot was on vacation, sick, in training, etc.

If the operation has TWO PICs, then the operation usually has a day hire pilot fill the role of SIC so that the airplane can keep moving, and keep the overhead costs lower than having a seldom needed standby pilot.

This allows each full time pilot some off time without grounding the airplane.

As far as flight authority, that is traded between the two pilots on a day by day or leg by leg basis.

The two pilots involved must have a good working relationship for this kind of operation to go smoothly. That is why hires are often done on a known basis instead of off the street.