As that "thought exercise", a few weeks ago I sat down with a Flight Standards guy and had this very discussion. He agreed that the P135 PIC (on the dispatch/manifest, etc...) could log 'Acting PIC' while the SIC could log PIC as sole manipulator on P91 legs. A co-worker, and former ASI, disagreed with him. We went back to the office and did some more searching and my co-worker has now changed his opinion on what he found to agree with the Flight Standards guy. I didn't keep a copy of what he found which changed his mind, but I'll ask him about it. Just another datapoint in this on-going debate.