First of all, you didn't kill the yeast. What sorbate and campden do is inhibit reproduction of the yeast. Once fermentation is completely over, you can add the sorbate and campden. One campden tablet per gallon, and 1/2 tsp of sorbate per gallon. Dissolve that in some boiling water (1/4 cup or so) and then add it to the receiving carboy. Then rack the cider/wine into it.
Wait a couple of days, to ensure that it's add some time to sit. Then, you can sweeten to taste.
Adding them together won't work, as you've found out. Also, now that fermentation has restarted, you'll have to wait until it's finished and try again. Adding more now won't stop fermentation once it gets going.