Twins of Evil was the third film in Hammer's "Karnstein trilogy," following Les Passions des vampires (1970) and La soif du 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.
According to Playboy Playmate and actress Madeleine Collinson, the occurrence of twin births was incredibly common for their family noting that nearly every woman in the Collinson clan had given birth to twins "one time or another." In fact, Madeleine and Mary's mother, a former model, would later give birth to a second set of twins.