Happens all the time

A guy starts talking to two women in a bar, they turn out to be Siamese twins, and they wind up back at his apartment. He makes love to one, and then starts to work on the other. He realizes that the first one might get bored watching, so he her asks what she'd like to do. She says, "Is that a trombone in the corner? I'd love to play your trombone." So she plays it while he screws her sister. A few weeks later, the girls are walking past the guy's apartment building. One of the girls says, "Let's stop up and see that guy." The other girl says, "Gee...do you think he'd remember us?"