@Weeg makes a good point too. But regardless of your plan with the healthy one that’s giving you trouble, it’s only humane to try to find a way to dispatch the unhealthy fox, or contact someone that can catch and treat it, mange can be dealt with I believe.
Sounds like mange or possibly rabies on those ratty looking foxes. I’d be careful when going out to chase them off, as well as making sure your dog stays safe. I’d say that Fox needs to be dispatched, it’s got a taste for chicken now, it won’t just move on.