(Side note - I could never really comprehend how there logically could be two PICs on board one aircraft...)
There is only ever one PIC: the person with the final authority over the flight. However, per FAA rules, there is a distinction between
being the PIC and
logging PIC hours.
I.e., an appropriately rated pilot who is not the PIC may still
log PIC hours in some circumstances, most notably when that pilot is "the sole manipulator of the controls of the aircraft". See FAR 61.51(e)(1).
For example, a rated PPL holder receiving dual IR instruction from a CFII may log PIC when s/he is the "sole manipulator" of the aircraft. Here the CFII is the PIC and has the "final authority" over the flight; The PPL pilot is not the PIC but may still
log PIC hours as the "sole manipulator".