They do make an "Air Switch" that has two plugs on it that switches back and forth like a three way. Hit the switch, and the unit takes power supplied to a Hot water dispensor, and turns on the Disposal. Hit the switch again, same in reverse...
However, I wouldn't think of using it, or a 3-way switch like that for a Dish Washer. It would become a nusiance to the home owner to reset the DW cycle everytime the GB was used. And you would have the HO curse your name every time they remembered that you cheaped out and didn't run a circuit for the extra $25 dollars of material and minimal effort. (Not that 'they' would know that, but you would...)
Common practice to just run a 3-wire to the sink for the both as a give-in. Priced that way off the bat. That way you're covered if they up-grade the appliances after rough, or anytime in the future.
