There Can Only Be Two (Labyrinth of Evil)

Thirteen years earlier, when Obi-Wan Kenobi had fought and killed a Sith on Naboo, had he killed a Master or an apprentice? The question was rooted in the belief that the Sith, having essentially defeated themselves a millennium earlier, had learned that an army of Sith could never stand, and that there should be only two at any given time. lest a pair of apprentices conspire to combine their strengths to eliminate a Master.

More a doctrine than a rule; but a doctrine that had managed to keep the Sith order alive, if well concealed, for going on a thousand years.

But the horned and tattooed Sith whom Obi-Wan killed could not have trained Dooku, because Dooku had still been a member of the Jedi Order then. As clouded as the dark side made some things, there was simply no way Dooku could have been living a double life within the walls of the Jedi Temple itself.