There is only ONE PIC on an airplane. He is the one who signs the dispatch release (Part 121) and logbook.
FAA regs allow you to log PIC time when you are the PIC OR when you are manipulating the controls. In general, though, when airlines ask for your PIC time, they ONLY want that time when you are the actual PIC, not just the pilot flying (PF). So, if you are looking for an airline job, and you show an excess of logged PIC time for your circumstances, you may have a hard time explaining...
Personally, I ONLY log PIC time when I am actually the PIC, NOT when I am a relief pilot and PF. You may want to develop a logbook spreadsheet that details all the different types of pilot time you might want to log...