"The girls were Siamese twins with perfect upper bodies joined at the waist and sharing one set of hips and legs."

Electra and Iphigenia are, judging by Olympia's description, a pair of conjoined twins of the Parapagus type which is to say fused side-by-side with a shared pelvis.

Chang and Eng
Public DomainChang and Eng

Conjoined twins are sometimes known as Siamese twins. The term was originally used to describe twins Chang and Eng Bunker who were born in Siam (now known as Thailand) in 1811. In 1829, they were "discovered" in Siam by British merchant Robert Hunter and exhibited as a curiosity during a world tour. They later travelled with P.T. Barnum's circus.