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There is only one PIC and the other is SIC or Co-pilot or P2.
PIC is about more than poling the machine around. It includes pre-flight planning and execution of the mission in it's entirety. So if you take the controls as 'PICUS' for 20 minutes out of flight that was planned,approved and signed for by the other guy or gal?
You are not in any way performing as a PIC, you are just handling the a/c in your role as a co-pilot.
PICUS is for use when under a formal upgrade program and only when you're flying with a trainer.
Becoming a trainer requires a regulated course itself (except line training) and the amount of trainers is usually limited by the authority to ensure that the approved trainers have enough workload to maintain currency in the role. When the time comes to upgrade, the flight is conducted under completely different terms; with an approved trainer giving more latitude and demanding more information about why you make the decisions you do!
I use the word trainer to cover the international nomenclature for Check Airman-TRI/TRE etc