Even though in the OT Vader and Emperor are both plotting against each other to turn Luke to their side and kill and then turn on the other, if there is a rule of two, it shouldn 't have even been talked about?
Think about it, if there is a rule of 2 established by the Sith, then why would The Emperor and Vader talk in ESB about turning Luke to their side, even though they are both lying to each other? If there is a rule of two, and they supposedly are saying here is the third wheel to help us rule the galaxy, then it exposes that one of them has to go, and instead of plotting like The Emperor and Vader do in ROTJ, it is talked about in ESB?
Before the PT there was no rule of 2 crap, so even though they The Emperor and Vader are plotting against each other in ESB & ROTJ, it is believable that they both feel the other is genuine since there is no limit on the amount of sith at once. But if there are only 2 siths allowed at once, then the conversation in ESB now make NO sense, because they are both 'showing their cards' before their motives are finally shown at the end of ROTJ.
Another plot hole that the PT created.