According to the scant records I found in the Tachibana house, the origin of the Kiryu and Tachibana houses being referred to as the "Twin Houses" lies in their constructions. Namely, they're meant to appear as reflections of one another, like two sides of a mirror.
That is, with one exception. When I read through the blueprints for the Tachibana house, I couldn't find any equivalent to the strange, mysterious space in the Kiryu house that I discovered previously.
The writings I've seen indicate that after a pair of twins from the Kiryu clan fled the village ages ago, a secret space was erected. Perhaps that space is what I found in the blueprints.
There are indications that other such concealed locations exist within the village, too.
Near Minakami Cemetery is the decrepit remains of a big tree. Inside that cavity is a gravesite that is seemingly dedicated to twins who were unable to fulfill the Hidden Ceremony.
It's a secret tomb, meant to be hidden not only to outsiders such as us, but even the villagers themselves.
The existence of this grave suggests to me that instances of the Crimson Sacrifice Ritual failing are deeply feared among the villagers. If Ryozo's two childhood friends were unable to carry out the Crimson Sacrifice they may well be buried there as well.
As if all that wasn't enough, records also indicate that there is a path from the Old Tree that leads to a place referred to as the "Umbral Mound."
The word "umbral" in that name would suggest a connection of some sort to the Shadow Ritual that takes place when the Crimson Sacrifice Ritual goes awry.
While records surrounding the Crimson Sacrifice Ritual only survive in bits and pieces, investigating this hidden ceremony may uncover the true nature behind the Crimson Sacrifice Ritual itself.
As they say, where there is light, there is shadow. Thus, I shall next set my investigatory sights on the Umbral Mound.
Note
The line about twins from the Kiryu clan escaping may be a mistranslation. The Japanese text appears to be referring to a hidden room that was constructed in the Kiryu house ("house" and "family" can both be written with the same character, 家) after some twins escaped. This is likely referring to the incident described in Bloodied Note, where a pair of twins from the Osaka Family tried to escape and the head of the Kiryu family at the time took his own life in what would become the Hidden Room.