The final film in the Star Wars Sequel Trilogy introduced the concept of a Force Dyad to explain the unique bond between Rey and Kylo Ren, but the sudden revelation left a lot of continuity issues in its wake.
As a conclusion to the eons-long conflict between the Jedi and the Sith, The Rise of Skywalker put forth several new concepts surrounding the Force. One of them, as displayed by Rey, was the power to transfer your life essence into someone else, reinvigorating them in the process. The other, meanwhile, speaks of a prophecy much like the Chosen One, which actually makes Force Healing possible in the first place.
In fact, according to Palpatine, a Force Dyad is a power “like life itself, unseen for generations.” How Darth Sidious managed to exploit that bond, though, is a plot hole that’ll forever haunt J.J. Abrams’ concluding flick. But if we take a look at the official novelization for Episode IX, we might be able to work out why the Sith cared so much about the ancient prophecy.
Apparently, these dark side practitioners had tried to create a Force Dyad for centuries. This is what ultimately led to the Sith Rule of Two, though as the phantom Emperor explained, that was only “a pale imitation, an unworthy but necessary successor to the older, purer doctrine of the Dyad.”
In his own words, Palpatine was “all the Sith,” meaning that he had access to the wisdom of those who came before him. He even boasted about his mastery over death in Revenge of the Sith. So, with all of that power, why couldn’t he develop a Force Dyad with Darth Vader?
Well, Star Wars has made it pretty clear that a Force Dyad is a naturally occurring phenomenon. Though even beyond that, the principle itself requires both the light side and the dark, something that the Sith obviously never considered throughout their reign of terror across the galaxy. As such, it stands to reason that as long as Anakin channeled the dark side, Palpatine couldn’t form a Force Dyad with him, despite his best efforts. And the galaxy was better for it.