This sort of question keeps coming up. I think that the answer is simple. Assuming that we are talking about a spamcan type light single then it will be certified for single crew operation. That is (other than for instructional flights) there can only be one pilot logging time, everyone else on board is a passenger. So you need to decide who is the pilot and fly within the contraints of his licence / ratings.
What you can do is swap pilots mid flight so that for example a non instrument rated pilot could hand over control to his IR rated mate who is sitting in the other seat if the flight enters IMC. But at any one time there is only one pilot and you log the portion of the flight flown as PIC accordingly, the rest is unlogged.