I don't claim to know it all, just a bit about recruitment and enough to spot a rigged logbook at 20 paces. You can't have two pilots in command simultaneously, regardless of what they may claim. PIC versus ICUS etc is seen for what it really is in this part of the world. Go and look at the dictionary definition of 'command' if you doubt.
Part 91? WTFIT? Is it here in Oz already? I am assuming that the original question was asked with relevance to the local situation, not what prevails in the USA or Europe.