Well Sidieous didn't believe in the rule of two. He followed his own rule of one. Before Disney nuked the EU he trained a couple of force users in the Sith way. The whole Sith thing is convoluted. I mean what would they do if another dark force user popped up and claimed he was the dark lord of the sith?

As far as Jar Jar being a secret Sith. It would have been a damned fine twist to explain the character.