There are several ways to log P1 for both pilots.
One of them is when an instrument rated pilot practice IFR (to gain IFR currency), he/she wears a IFR hood and simulates IFR condition while the other safty pilot provides look-out. In this case, they can both log P1 (not sure which type of P1 though).
Another one is when you are an airline pilot (or at least comerial). Captain and First officer share legs for experience. e.g. outbound leg, PF:Captain is P1 while PNF: FO is P2, inbound leg, PF: FO is P1US while PNF: Captain is still P1. In fact the captain is still in comand regardless who is flying.
(not entirely sure about this case)