That's the cool thing about playing a game. You get to decide what happens in your universe. Perhaps in canon there were only two, but in OUR star wars universe that you and I are creating as we go along, perhaps there is 5? Or 10? Or maybe one pureblood sith jedi set out to create an entire legion of them?
Either way, this is a "choose your own adventure" setting, and that change allows us choice.