Twins of Evil was the third film in Hammer's "Karnstein trilogy," following The Vampire Lovers (1970) and Lust for a Vampire (1971) in a series loosely based on Sheridan Le Fanu's "Carmilla." These pictures were produced quickly--the trilogy's original UK release dates range only from October 1970 (The Vampire Lovers) to October 1971 (Twins of Evil)--and they were lurid even by Hammer standards, bloody and relatively steamy, with an emphasis on heaving bosoms and vampire-enhanced girl-on-girl sexuality.
Mary Collinson and Madeleine Collinson were from Malta and still had strong Maltese accents. As they had done with other foreign actors, Hammer simply had their dialogue dubbed by British actresses