In my experience (a couple of ginger kids at Uni; 'Goodbye, Gemini') there is always at least one evil twin. It's a tried and tested horror formula, of course, and some bright spark at Hammer had the idea of reviving it using the double charms of the Collinson twins, Madeleine and Mary (or is it the other way around?) Collinson, who had recently made their parents proud by being the first identical twin Playmates in history.
The third in Hammer's 'Karnstein' trilogy, the action in 'Twins Of Evil' actually takes place before 'The Vampire Lovers' and 'Lust For A Vampire', so is technically a prequel although, as you might expect, the continuity is all over the place.
Unsurprisingly, the twins flash a fair bit of flesh, are dubbed, and prove something of a handful for their upright guardian, Peter Cushing, a Puritan and Witch Hunter. It's a good role for Peter, and he makes the best of it, as does ex-Powell & Pressburger star Kathleen Byron as his equally messed about wife. Set your expectations to 'reasonable' and be prepared to be pleasantly surprised.