They sell piggy back dimmers that you can just plug the uth into. No wiring involved. They look like this:
http://www.homedepot.com/webapp/wcs/...10000003+90401
Also, when you get a thermostat, you only need one. Even the cheaper ones come with two plugs so you can control both tanks with the same unit. The more expensive units have space for up to 6 heating elements.