Even within the confines of the EU (which is where most of the Sith lore lies), which doesn't really count, the Rule of Two was not the origin of the Sith. Darth Bane established that rule, some time after there were already Sith Lords.
The Sith Lords were originally Dark Jedi (whatever the hell that means - again, it's EU, which doesn't matter).
So there is no reason why there couldn't arise other Dark Jedi who found a new Sith Order of some kind. And if so, there is no reason to think that they would have to continue with the Rule of Two. That obviously didn't work out so well.