Sisters (1973) recycles the cut-apart plot, only the surviving twin Margot Kidder (above, right) periodically adopts the homicidal persona of her dead sibling, Dominique, all the better to butcher unsuspecting lovers. Ironically, birthday cakes bring out her worst side. See The Addams Family (1991), where Gomez's old flame turns out to be 'twice the woman' Morticia is.
Frank Henenlotter's Basket Case trilogy (1982) has the last word on the phenomenon, offering the relatively sane Duane (Kevin Van Hententryck) and his grossly deformed and demented brother, Belial. They too are telepathically linked, seeking revenge on the doctors who split up the act, tossing Belial into a rubbish bin. Basket Case II has Duane unable to cope with his isolation and sewing Belial back on to his body.