This actually happened -- Ron, a college guy, was dating Susan who was the identical twin of her sister, Sharon. One evening the girls decide to see if they can switch out with Ron -- in the middle of the date -- without him knowing it. Whatever happens is fair play between identical twin sisters.
This is part of the Libertine series. Part of the Massachusetts Family Love universe
This is a spin-off from the Hank serial, which begins to describe life for Reginald, Sally's narrow-minded ex, who is turned into a sex slave and gigolo for Sally's half-sister Candy and Candy's half-brother/beau Lewis.