When FedEx first started flying double crews KIX-MEM the Captain that had FDX18 on his trip was PIC, the Captain with FDX7018 was the Relief Captain. On one of these trips the PIC was junior. When the crew was alerted they were informed that there would be a jumpseater. Actually a FAA Maint. Inspector. The senior Captain had a real problem with this. There was not enough room, the cockpit was too crowded blah, blah. We had another flight that also went to MEM via ANC a few hours later. The inspector agreed to take the later flight. All was fine until the inspector's boss found out about it. The two Captains were called in to explain why the FAA inspector was bumped. The senior guy, the relief Captain, the only guy that had a problem with the 5th guy in the cockpit said "Hey, I was not PIC". The junior Captain, the PIC, had his ATP suspended for two weeks as punishment for bumping a FAA inspector. It was after that event that the most senior Captain was PIC. The senior Captain system then caused problems when they started building trips that were D/H MEM-CDG, operate CDG-Subic, D/H Subic-KIX, operate KIX-MEM. Some had a real problem with this system.