“Always two there are…” Except when there isn’t

I think this is likely. That could have meant that the Sith work in pairs and not in groups any larger. If we go by the movies alone, there is no Rule of Two. It was only books, I believe, that established that there are literally only two Sith at a time.

Someone else stated that this Rule of Two didn't exist when Jedi came out. So as far as the Emperor telling Luke to kill Vader, I think Vader likely knew this was a ploy to get Luke to turn, which is why Vader didn't say, "Um, excuse me?" Vader knew the tactic and played along.

If you read the excellent Plageius book, you'll learn that the Rule of Two was abolished when Palpatine became an apprentice. That would make Luke's addition to Team Sith a non issue. Of course, the Plageius book was also written long after Jedi came out.

 

The Rule of Two By Darth Bane is curious device. I have wanted it more explored in cinema and games. From what I understand of the Sith, the goal is power. If s more powerful apprenrice presents him or herseld, the weaker apprentice is discarded as in ROTS when Palpatine urged Anakin to slay Darth Tyranus (Dooku). At some piint the Sith Master is subject to being dethroned, as in the case when Darth Malgus slayed his master at the Battle of Korriban. Sith are like lions, if one of their own shows weakness, the stronger lion makes the kill and takes the pride as ling or in this case Sith Lord: