"Back To Back" joke

Jon came home in great excitement and said to his wife, "Judi, my love, you'll never believe it, dear, but I've discovered an entirely new position for lovemaking."

"Really," she said, interested at once. "What is it?"

"Back to back," Jon replies

"But that's crazy. We can't do anything back to back."

"Yes, we can," he says. "I've persuaded another couple to help out!"