There's an occasional coyote around my rural apartment but Autumn never goes outside alone. I only let her go potty in an open area at night where I can see something approach before it gets to her. I think just keeping an eye on your dogs is a better plan than getting a big dog to be a coyote scare tactic. Keeping prey animals from setting up shop around your home would probably help to keep coyotes from targeting your place too. I know the millions of rabbits around my apartment bring the predators in. You may also consider fencing a smaller yard area for the dogs to be out alone at times without worry.