TMBTM said:

And the "rule of two" is stupid.

Well, (Lucas speaking mode:) even in the original trilogy there is an echo of that "rule of two", when Vader asks Luke to join him so they can destroy the Emperor and rule the galaxie as father and son. Vader knows that once Luke is converted to the dark side, the Emperor will not need him anymore.

Yeah, but Vader's just sick of being the Emperor's puppet - he's power-hungry at this point and wants to rule on his own, not be the underling of someone he knows will never relinquish power. He knows, however, that he's not powerful enough on his own to stop the Emperor - but with Luke at his side, they may win. It has nothing to do with the "rule of two."