Dead Ringers (1988)

Beverly and Elliot Mantle are identical twin brothers who work in a joint practice as award winning research and practicing gynecologists. They both have been fascinated with the female reproductive system since they were children. Despite being physically identical, they are extreme opposites in personality, Elliot being the extrovert, Beverly the introvert. As such, they live a symbiotic life, with one's strength being the other's weakness, both personally and professionally. The two brothers, under the guise of being Beverly alone, begin a sexual relationship with visiting actress Claire Niveau, who comes to see Beverly professionally about a physical abnormality which may prevent her from being able to conceive. Claire is unaware that Beverly has a twin brother let alone that she has had sex with him. When Claire finds out, she is initially disgusted but continues to have a relationship solely with Beverly. Beverly's growing obsession with Claire and her physical abnormality ultimately shows how symbiotic and intertwined the brothers' live are and the fact that one can't live without the other.