Chang & Eng Bunker were cojoined "siamese" twins that worked as freaks in P.T. Barnum's circus sideshows; they had 21 children after they married two sisters. Pretty fucking strange stuff, right? Well, hold on, because this gets even weirder. As the wikipedia notes, the twins were only joined at the sternum & shared a liver, so even by 19th century medical standards, they could have been easily separated, but never were.
http://en.wikipedia.org/wiki/Chang_and_Eng_Bunker