You can't really halt fermentation with campden and sorbate. Neither kill yeast, but the sorbate inhibits yeast reproduction, and works better in the presence of sulfite.
What that means is if you still have tons of yeast in suspension, adding the sorbate and campden won't stop fermentation. That's because the yeast don't need to reproduce any more- there are plenty to ferment.
Stopping an active fermentation is like stopping a freight train. You may be able to do it, but if you just dump sorbate and campden into the cider you'll probably just get some off-flavors and not a stopped fermentation.
It's more likely to work well if you chill the cider at the desired level of fermentation. Wait at least several days, and it should start to clear. After it's clear, you can rack off of the lees (where much of the yeast is) and into a solution of campden (potassium metabisulfite) and sorbate. I'd go with 1 campden tablet per gallon (50 ppm free SO2) and 1/2 to 2/3 teaspoon of sorbate per gallon. Wait and see if fermentation did indeed stop, and then bottle in about a week if there are NO signs that fermentation will restart.
A couple of things- sorbate does have a taste. Some people find it unpleasant. Instead of using the campden/sorbate, some people like to bottle at a specific SG and then bottle pasteurize. I haven't tried it, and I'd be afraid to do it, but Pappers has a great sticky on how he does it.