You use it when you mean something that is true but not LITERALLY true. In your example there's not actually two laws written in the law books--it's just that this is the way the law works in practice. So you use a phrase like "so to speak" (sometimes you'll see "as it were" to communicate the same thing) in order to show that you know that what you're saying isn't "literally" true. Example: "By explaining this I'm being your teacher, so to speak." (I'm not actually your teacher in a formal sense, but I am currently teaching you something. So I add "so to speak.")