Actually the Rule of Two predates Darth Bane by quite a bit. (Although he did bring the practice back to the fore and give it a name.) Darth Rivan (not Revan) was the last known Darth before Señor Bane. Rivan settled on the world of Almas and constructed a fortress containing traps based on adherence to the Rule of Two. [spoiler]Rivan did base his name off of historical texts on Darth Revan. They are two separate people, however, and it's not a typo.[/spoiler] These traps were inspired by Darth [i]Revan's[/i] teachings on never training more than one apprentice. So while Bane [i]did[/i] institute the Rule of Two in the new Order of Sith that he created the practice itself was based on much earlier teachings. He simply brought it back and enforced it. Harshly.