To be fair, who would have policed the policy either way? If there's only two, they're the only two who could. And if it was just down to one, who'd tell him no? Is there a secret council of Sith Who Aren't Sith But Policy All the Silly Policies They Apparently Have?
Maybe THE SITH? Perhaps they're a species that stays on their own world, and only allows their secret sorcerous knowledge of the dark side of the Force to be known by two outsiders at any given time - a Master, and his Apprentice. And if they find out that anyone else is practicing their arts, they will come out just long enough to hunt them down and eliminate them.
I mean, that goes completely against what we knew from the EU, but since that's all out the window, why not?