Adding to Samus’ comment, it’s also an EU thing. Yoda mentions in Phantom Menace that there are always two Sith, the master and the apprentice. This is because of a rule set up around 1000 years ago by Darth Bane. At his time, there were hundreds of Sith with academies teaching new Sith kinda like the Jedi do. By the end of his book trilogy, he set up the new pattern of exactly two Sith at any one time - a Sith master to embody the power of the dark side and an apprentice to covet the power. The apprentice proves they are ready to take over by killing their master, and thus the Sith only ever grow stronger. This lineage continued until Vader and the Emperor, so they were both expecting Vader to eventually challenge the Emperor
EDIT: Although in some of the books which are probably EU now, I feel like Palatine was so cocky that he thought he was the chosen one of the Sith, called Sith’ari I think, and so he would always be the master and Vader always the apprentice
Adding to Samus’ comment, it’s also an EU thing. Yoda mentions in Phantom Menace that there are always two Sith, the master and the apprentice. This is because of a rule set up around 1000 years ago by Darth Bane. At his time, there were hundreds of Sith with academies teaching new Sith kinda like the Jedi do. By the end of his book trilogy, he set up the new pattern of exactly two Sith at any one time - a Sith master to embody the power of the dark side and an apprentice to covet the power. The apprentice proves they are ready to take over by killing their master, and thus the Sith only ever grow stronger. This lineage continued until Vader and the Emperor, so they were both expecting Vader to eventually challenge the Emperor
EDIT: Although in some of the books which are probably EU now, I feel like Palatine was so cocky that he thought he was the chosen one of the Sith, called Sith’ari I think, and so he would always be the master and Vader always the apprentice