Some might not think it's possible but...
:
"The story of Chang and Eng Bunker, the “Siamese Twins,” so called because they were from Siam (now Thailand). Chang and Eng were joined by just a bit of liver and some skin. One April day in 1843, Chang married Adelaide Yates, while brother Eng married sister Sallie Yates. Based on the fact that Chang and Adelaide had 10 children, and Eng and Sallie, 12, it’s fair to say the brothers had sex."