Two (Stereo?) Bullwhips
You’ve probably got the idea already. One mistress loved using the bull whip and was ambidextrous. Well, she had me lay down on the bondage bench, strapped me down with a fine collection of leather straps, then put a folded towel over the middle of my back.
Then taking up two bull whips, one in each hand, she proceeded to crack them over my butt in turn, crack-crack, crack-crack. It was painful but even though it was about 10 years ago, I seem to remember not all that painful. A good hard caning made far more of an impression.
One thing about this mistress that was concerning though. When she was getting into it, really getting into it, she had a habit of taking a really deep breath. You could hear it distinctly. At the time it sounded a bit like she was losing control, going to go maybe too far in the session.
Not long after this session she left my city. She didn’t like the weather all that much. She wanted to live somewhere warmer.