In the rather bizarre situation described I'd firstly try to talk the customer out of it, but if he insisted, for the reasons you gave yourself, I'd designate one the pilot and make sure the other understood that he was just a passenger in uniform out for a jolly. (pax handler and gopher perhaps). The hazard is two "single" pilots feeling they need to act as a crew. Don't let it happen. Stick to one pilot and a pax in uniform only. DO NOT try to operate the aircraft as a "crew"as it is outside the pilot's experience.
Its a very uncomfortable situation for the pilots and imo has the potential to be quite hazardous.
Best answer, educate your customers.