I believe the original film focused almost entirely on the dyad. It’s why there are lingering threads of the two of them being on the throne together in the theatrical release despite no such footage existing. Kylo was probably focusing entirely on how they’ve always been the same person through the Force and it’s their destiny to rule together (Sith prophecy - the rule of two manifested into what it was always meant to be), and how Palpatine is willing to give it to them because of how weak he is. But then the whole “your coming together will be your undoing” twist happens.
That’s actually true, to a certain degree. According to the infamous Reddit plot leaks from an earlier stage of production, the Rey Palpatine twist was still going to be there, but it was going to be supplemented by a much larger focus on the Dyad, with Palpatine even telling Kylo about it in his introductory scene:
Palpatine begins to reel Kylo in and subsequently tells him of something known as a “Force Dyad”. The Force produces two incredibly strong users and when they unite together, both become stronger than either one could be on their own. Palpatine believes Kylo Ren and Rey to be the two halves of this dyad and shares this knowledge with Rey. (Note: I previously wrote that Palpatine wanted them to become “master and apprentice” since then this has been clarified to me as Palpatine seeking the two members of the Dyad) Palpatine claims that he seeks the Dyad because he is weak and dying and he wants the Dyad to rule together once he’s gone. Palpatine orders Kylo to bring Rey to him and turn her so that the Dyad will be a strong, unified force of darkness.
By the end, of course, it turns out that this was a ruse, and Palpatine drains the Dyad’s energy once both members show up on Exegol.
If I had to guess why they didn’t go with this in the final movie, it would probably be to give Kylo more agency, rather than just blindly following Palpatine’s orders. That is a legitimate concern, but I feel like it still would have contributed a lot to the movie for the Dyad to be part of Palpatine’s plans from the very beginning. That way, the life-draining scene at the end would actually feel like Palpatine’s traditional 4D chess, rather than him just happening to stumble across the power he needed.