Sorry to be nitpicky, but campden does NOT kill wine or ale yeast. Wine yeast is tolerant of sulfites, that's why they're used in winemaking and cider making.
Everyone seems to say campden does kill yeast, but in the amount you'd need to do it, the beverage would be undrinkable. In the amounts we add, generally at 50 ppm or so, the yeast won't be even stunted let alone "killed".
Sorbate doesn't kill yeast, either, but it works to inhibit reproduction. so, if you truly want fermentation to not restart, you use sorbate after fermentation has finished. You rack the cider/wine/mead off of the lees, and into a sorbate/campden mixture. Sorbate works better in the presence of sulfites, that why you use campden also.
Once an active fermentation is going, it's almost impossible to stop at a desired amount of sweetness with campden and sorbate. You can chill the beverage, which causes the yeast to become dormant. That, combined with sorbate and campden MIGHT halt fermentation. But it might start up again once the cider warms up. Most people find that it's more practical to let the beverage ferment out completely, then stabilize it (with sorbate and campden) and then sweeten to taste.