The rule of two doesn't make sense in the regard that as a Sith, I would simply never take on an apprentice. I would go about my business on my own. The less energy I have to expend looking over my shoulder, the more I could focus on the tasks at hand.
In the short run though, you have a reliable lieutenant to do your dirty work for you. That happens to be a Sith. You can just as easily manipulate / dispose of your apprentice, arguably moreso than they can of you. Example: Sidious and Dooku.