Sidious tells Yoda in ep. III that Darth Vader will become more powerful than either one of them. Meaning that at the time he expected Vader to surpass and kill him, and he was delighting in that. He even tempts Luke to kill him, believing whatever Luke decides, Sidious wins. It's also my understanding that he planned on being the final Sith Lord and never dying anyway.
Besides the Rule of Two is quite simple. "Always two. No more, no less. A master and an apprentice." That's it, and the emperor observed it.