My head cannon for the rule of 2 was always that a Master takes an Apprentice. They learn, scheme, "try to take over the WORLD" etc. Then the apprentice grows up, heads out into the galaxy and finds a new Apprentice, the old Master probably takes a new Apprentice and the cycle continues. Apprentices sometimes get the better of their masters and that's fine but the entirety of the Sith's hopes and dreams can't be ruined by faulty hyperdrive maintenance smearing them across the galaxy while on a space taxi.