i can only quote someone else since i don't know the answer myself... on that note i regularly read that sorbate works better with sulfite but i have never seen a biological explanation. i tend to trust the experience of those who say it though. this is from oft-cited jack keller's website:
Sulfite, by itself, does not actually kill off the yeast. Instead it creates an environment increasingly hostile to yeast and deadly to most other microorganisms such as bacteria. Wine yeasts, however, can be quite tolerant of sulfites. For that reason a stabilizer is also added to the wine.
Potassium sorbate, sold as a chemical or behind a product name such as Sorbistat K, is a commercial wine stabilizer that should be used in conjunction with Campden. In other words, it works better with sulfites present than without, and it works better than sulfites alone. Potassium sorbate disrupts the reproductive cycle of yeast. Yeasts present are unable to reproduce and their population slowly diminishes through attrition.
Potassium sorbate is added in the amount of 1/2 teaspoon per gallon of wine. Sorbic acid results and stabilizes the wine. Usually the crushed Campden and potassium sorbate are dissolved in a cup or two of the wine to be stabilized and stirred thoroughly. Allow the stirred wine to sit a few moments and look for small white lumps of undissolved powder. If present, continue stirring until the wine is clear without any undissolved lumps. This is then added to the larger batch and stirred in well with a sanitized glass rod or wooden dowel.
I've also heard that if your cider/wine is going through malolactic fermentation, the addition of P. sorbate alone will produce some funky geranium smells and tastes. It ruin's the batch I'm told from a wine guy I know. Sulfite added before sorbate supposedly stops that process from happening. No firsthand experience here either, so take it with a grain of salt.