Lady Scarlet wanted to make sure this guy was the sort of person she needed to hire. And she went ahead to test his pain threshold. That is why she chose to ballbust him. She closely watched out for how he reacted to the pain and she knew that he was the guy she wanted as he seemed to have a high pain threshold. He did not cry even though it was painful.
Mistress Kat wanted to ballcrush this guy because he was a liar. The mistress had to make sure that he stopped his lying tendencies and that is why she used her high heels to trample his balls. The mistress inflicted a lot of pain on him and she knew, by the time she was done, that he would never be a liar again. And she was right as he never lied again.