Reading this gave me another (unrelated) idea.
Often times when ballbusting, the slaves act annoying and close their legs, hiding their balls.

I was thinking of attaching two bands just above the knees of both of their legs.
The bands would detect the distance between each other and if they get too close (let's just say, for example, if the slaves close their legs to hide the target from the mistress), it would shock them bad. It would keep shocking at specific time intervals like 1 sec or so, until they pull their legs apart again.

With modern technology they shouldn't even be that hard to make.