Very well thanks mate. I hope all is well darn sarf. Get that project done or Roger will be after you!
Every time questions like these come up, I shake my head. As with most thing in aviation, a good dose of common sense will tell you the real answer. Ask yourself the question, Why do I want the answer to this? Is it because I'm trying to build hours on the sly? Or is this a genuine situation like the mentioned by skyyacht?
In reality it is very simple. A single crew a/c can only have a single pilot (unless there is an FI giving training onboard, but even then, there is only one captain. For example I cannot log the time that I send a student solo, though I do get paid for it.) so in most circumstances, only one pilot can log the time, basically, the person who signed for the a/c and who in the event of a disaster will be deemed PIC in absentia. You sign for it, you log it. It gets broken, you answer for it, not the "passenger."
As before, not rocket science, anything else is just someone trying to get around the rules of common sense for their own gain.