The Sith Rule of Two purpose was Darth Bane’s philosophy to ensure the survival of the Sith by eliminating infighting and so that they could live in the shadows to covertly grow the dark side and beat the jedi. It’s purpose was not to create some stupid force diad even if some of them had that in mind and tried it like Plaguis did. It was just a side project not the main purpose of the Rule of Two. That always remained the same.