Omega-Prime, the rule of two is not an old tradition of the Sith. The oldest tradition had many Sith -- many apprentices, until they began to fight and kill each other. With powers attainable by tapping into the hateful energies of the dark side, it was only a matter of time before the order self-destructed. Internecine struggle by power-hungry Sith practioners dwindled their numbers. One Sith had the cunning to survive. Darth Bane restructured the cult, so that there could only be two -- no more, no less -- a master, and an apprentice. Bane adopted cunning, subterfuge, and stealth as the fundamental tenets of the Sith order. Bane took an apprentice. When that apprentice succeeded him, that new Sith Lord would take an apprentice.