Re the original question, I am not sure, without checking, if the identity of the a/c captain can change during the flight of a single crew a/c (but I suppose it can if the Captain eats the dodgy fish and Doug McClure has to step forward from the back row and grapple with his 'Nam flashbacks, whilst George Kennedy yells encouragement at him from the fogged in tower below).
Back in the real world, if the identity of the Capt can change mid-flight, then PPL is the PIC for his bits, and logs that. PPL IR logs the instrument bit. What I am pretty sure about is that both can't log P1 for the same period. I belkieve that neither can log P2 because the a/c is single crew and the "co-pilot" is not exercising his licence privs as a necessary part of the crew.
If the Capt CAN'T change, then, assuming that the aircraft is a single-crew type, isn't the answer that the PPL logs the whole flight as P1, but does NOT log any instrument time. The PPL IR logs nothing at all. In this cases, isn't the IFR bit of the flight a bit, er, naughty?
I will go and look it up later, but, for chapter and verse, why not try posting the q on the Charlie Alpha forum and you'll probably receive a definitive answer from Irv Lee or one of the other regular instructor-contributors there.