In this kind of situation, the first prominent person with two heads would come up with a word now and then to describe his/her single-headed counterparts. Whichever one caught on with the masses would be the word.

Since that person would most certainly be Zaphod Beeblebrox, I'd expect the word roots to be from intergalactic slang, alcoholic drink names, or some combination of the two. The word would definitely not Latin-based or anything sophisticated.