You can fly right seat single pilot if you so wish in a single pilot aircraft with only you on board. In that case there would be no question as to who was PIC.
I have heard of flights with an instructor where one student has been observing, Where they have changed mid flight and the other student records the second portion of the flight.
as stated the only legal problem would be in the event of an incident where an insurance claim was made! then there maybe a question on who of the two actual qualified pilots was PIC as normally not LEGALLY the left seat is the PIC seat.
Pace