[calcapt]

This behavior, in addition to being unprofessional and a threat to safety, is disrespectful to the crew. They are compromising the entire flight over their bad behavior. One quick warning is warranted without further action; however, if any hint of additional bad blood is shown, they will both be replaced at the destination airport. Keep the relief officer in the back of the aircraft for the duration of the flight to avoid interaction. If any question exists about their ability to perform safely and professionally until you reach your scheduled destination, you should land at the nearest suitable airport and have them removed and replaced. This action will likely cost them their jobs. I can't imagine a situation where this behavior would continue, particularly after they have been warned about throwing them off the trip.
JMHO