Dead Ringers (film)

Separation can be a... terrifying thing.

A 1988 body horror by David Cronenberg starring Jeremy Irons.

It's a story about twin gynecologists, Elliot and Beverly Mantle, who run a brisk business treating infertile women. That, and they have a tendency of taking advantage of them, too. Being the more suave of the two, Elliot woos them, and when he's finished he pulls the old switcheroo and pawns them unsuspecting off onto Beverly.

Getting Elliot's sloppy seconds begins to tire Beverly, and he sets out to get a woman on his own. He makes the big personal leap of dating an actress he's taken a fancy to. Unfortunately, Beverly is a jangled bundle of neuroses, and when he sees the girl he's set his eyes on meeting with a coworker, Beverly jumps to the conclusion she is cheating on him.

Cue descent into delusional hell, wherein Beverly begins performing unnecessary operations with custom gynecological tools on women who he believes have "mutant genitalia". When the effects of his declining mental health spill over into the brother's practice, the board of health suspends Beverly's license. Elliot, who had always been sensitive to his sibling's fragility, leaves his practice and his current love interest to join Beverly, who has holed himself up in their apartment.

The twins then embark on a medical procedure to separate themselves for health reasons. Has it been mentioned that they aren't conjoined twins?

Tropes used in Dead Ringers (film) include:

Acting for Two: Irons plays both twins, often in seamless splitscreen.
Cow Tools: Beverly's custom-made gynecological instruments are shown on display, and they are quite nasty looking, but (thankfully) their utility is never made explicit.
I Was Told There Would Be Cake: Perhaps surprisingly, the eating of a delicious cake is an integral part to The Climax.
Identical Twin ID Tags: As the story progresses, Beverly and Elliot personalities diverge due to one of the twin's advancing mental instability. This is reflected in their appearance, with Elliot maintaining a healthy, well-tanned look while Beverly develops bags under his eyes, a sickly pallor, and a nervous twitch.
Mad Doctor: Beverly, after his mental decline.
Through the Eyes of Madness: Starts with Beverly, who begins seeing genital mutations in his patients that may not really exist. But the madness spreads to Elliot via twin osmosis. This leads to them using a scalpel to perform a "separation procedure", AKA a murder-suicide.
Twin Threesome Fantasy: Elliot orders up an evening of "entertainment" with a pair of high-class twin hookers called Coral and Mimsy. Taking it even further, he requests that one of them call him by his own name, and the other call him by his brother's name...