Yeah! You're right. Coyotes and foxes are enemies. Aren't they?As strange as it sounds to actually pay money for this, you can buy coyote urine to deter fox.
I remember the Environmental Educators at the local nature center saying that coyotes will either run foxes out of their territory or kill them, outright. So, if you see foxes or coyotes in an area, it's a safe bet that there won't be any of the other.
Using coyote urine that you buy from the hunting and outdoors store would probably work.
(Hunters often use natural scents like urine as cover scents to prevent animals from detecting their presence by sense of smell.)
Either that or figure out some way to encourage more coyotes to hang around.