From the personal writings of Darth Bane - dated 2654 ATC and preserved in the Jedi Archives

"The day the demon came is a day where the wheels of change began to turn. Kaan seemed so pathetically enamoured with him. He speaks well, for a demon. I have not forgotten his final words to me, spoken in secret beyond the reach of the others... 'Perhaps there is something to be gained from the Jedi,' he said, 'one master, one apprentice. No more, no less. Maybe the Sith need a rule of two as well.' I asked him why this would be better. 'I have seen how conquest and brute force ends for the Sith, and its never a good ending.' Said he. 'It might be better to conquer through stealth and subterfuge.' A rather militaristic tactic, but it does make sense. I have witnessed greed and in-fighting in excessive amounts within Kaan's 'equal-opportunity' Brotherhood. This weakens the Sith. So much power divided between so many limits us and creates division and strife, and such aspects will not topple the Jedi. A frayed cord makes for a useless whip...

...

"I have pondered this for a long time. I see now that the demon is right, two there should be. No more, no less. One to embody power... the other to crave it. That will be the new rule."

-End of excerpt