You shouldn't have to drink more water while fasting than normal. Less if anything as a lot of the water needs to assist digestion especially- or only!- of cooked and processed foods. These foods often have little water content of their own compared to raw foods, which when made a major part of our diet make drinking water pretty much unneccessary. I drink very little water at all now as when I fast I dry fast and when I'm eating I'm eating water rich raw foods. And I can assure you I don't suffer in either case.
I like that reference to the test with twins! I'll have to use that. :)