I was reading the Deep Crimson Butterfly futagomori ending on the FFTranslations website and came across this line
They look at each other. Yae's face alone looks slightly sly, and she reaches both hands out to Sae's neck. When Sae notices she smiles calmly and holds out her neck as Yae wraps her hands around it. It is their strangling game, their secret "becoming one" game they have played since childhood.
This line is interesting to me because Yae was always against performing the ritual and becoming one. Why do you think they had this game together?