Have you tried squirting Remington with water or throwing a puppy bomb (soda can filled with pennies) at him? I've been told the Feliway things work for territorial spraying in cats so I'm guessing since it is also meant to calm the critters that it may ease some of the tension, but I don't know that it will make Remington stop chasing Mystery. Does Remington get enough time outside to run and play and chase balls/toys? Does Mystery have claws so that he/she? can slap Remington on the nose to get the "point" across?