I did that once when I had the pleasure of a sub couple to play with: tied them together in 69 position and set them to eat each other out, with the promise of a thrashing for the one who came first, while I whipped them both to encourage them to try harder.

I cheated by fingering the woman to make sure she came, because it was more fun to leave the man to watch with a desperate unsatisfied boner while I punished his partner for "losing".