MM, If you have two pilots on board, and no passengers, the custom is that each pilot pays for his own leg(s) (where he/she is PIC) in full. No cost sharing whatsoever.
Good idea to make a few agreements, CRM-style, beforehand on distribution of tasks. Typically the PIC flies the plane, also in case of emergency, and the other pilot does radio and nav. But that's not a hard and fast rule. It also depends on the type of flight and whatever additional tasks there are in-flight.