Ok, as GL describes it:
"One of the themes throughout the films is that the Sith lords, when they started out thousands of years ago, embraced the dark side. They were greedy and self-centered and they all wanted to take over, so they killed each other. Eventually, there was only one left, and that one took on an apprentice. And for thousands of years, the master would teach the apprentice, the master would die, the apprentice would then teach another apprentice, become the master, and so on. But there could never be any more than two of them, because if there were, they would try to get rid of the leader, which is exactly what Vader was trying to do, and that's exactly what the Emperor was trying to do. The Emperor was trying to get rid of Vader, and Vader was trying to get rid of the Emperor. And that is the antithesis of a symbiotic relationship, in which if you do that, you become cancer, and you eventually kill the host, and everything dies."