Which of American Horror Story's Freaks Are Based on Real Circus Acts?

Conjoined twins have been a constant source of fascination, both in real life and pop culture. It's easy to see why two heads on one body, seen in this 1970 photo of these twins, might generate curiosity. They've appeared in other fantastical ventures, like Big Fish, as well as more true-to-life situations, like on Grey's Anatomy.