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?"