@tabitha @Felthry I've always read the Rule of Two as less "There Are Only Ever Two Sith" and more "alright, if you're a Sith and you aren't the master then you're the apprentice, trying to kill the apprentice to take their place, or trying to kill master *and* apprentice to exert your rule over the Sith instead."
After all, Darth Maul came back in the CGI Clone Wars, and he started building a power base to try and challenge Darth Sideous, starting by taking his brother Savage Opress (I cannot make these names up) as his apprentice and following it up by making moves on the Mandalorian government. It didn't work, of course, and Darth Sideous killed Savage and beat the brakes off of Maul.
@Felthry @tabitha That's what I'd like to know!