"To make the beast with two backs"
Othello
EDIT: I thought the phrase was of Shakespeare's making. Turns out there is an earlier use by Rabelais,
"In the vigour of his age he married Gargamelle, daughter to the King of the Parpaillons, a jolly pug, and well-mouthed wench. These two did oftentimes do the two-backed beast together, joyfully rubbing and frotting their bacon 'gainst one another"
How about "exchanging the codpiece for the salmon's tail."
