Is it really too much to expect two qualified flight crew members to decide who is to be the commander (yes, Chuck, think 'gold braid') in advance? Or for the operator to designate one of them as such?
If you have the conversation in advance then Beagle's comment
1. The FI will invariably be the Commander and will log PIC.
won't be too far from the truth, though there may be som circumstances in which an FI would agree from the outset to be a passenger.