You mean when Vader and the Emperor were talking about bringing in young Skywalker, they both knew that he would have to kill one of them? That does not track. They are too mutually enthusiastic about it for the "rule of 2" to be in effect.

"Good idea, bring in Skywalker and he gets to kill you!"
"Oh no, no, no-- bring him in, but he gets to kill YOU."
"Me?"
"Yes."
"Why me?"
"Because it was my idea."
"I don't care who's idea it was-- it only seems fair that he kills you because I've been a Sith longer than you."
"Then it's my turn-- you die. I live and Skywalker takes my place."
"I don't like it."
"Why not?"
"I just don't. Look-- let's decide who he kills later. Maybe nothing will come of this and all this bickering will be pointless, okay?"
"Okay-- but I'm saying he kills you."
"Ah-ah-ah. Stop that."
"I'm just saying."
"Don't make me use lightning on you. And don't forget you killed Padme."
"NOOOOOOOO! I'd almost managed to put that out of my mind, you jerk!"
"Sorry."