IMO if they have adequate prey and territory, they will coexist fairly well and not go out of their way to kill the "lesser" predator without exerting too much energy. I worked at a prison in Colorado that for years had several fox living around the perimeter, preying on the rabbits. A pack of coyotes moved into the area and I soon found two fox dead and did not see another one in the area until a couple of years later, and that was only a single sighting. This was on the edge of an urban area with limited prey and habitat. I know thats an isolated incident, but I think the same thing would happen anywhere that prey and territory are limited. Other than that, there were good populations of both coyote and fox in all other areas of Colorado that I have hunted. I have even called in fox at close to 12,000 ft elevation which I did not expect. I've called coyotes in around that elevation, but didn't think I'd see fox that high up.