I'd like to think that the two stories are actually connected. Koishi, having no clear, conscious mind, is basically a pre-development, subconscious-driven entity that has a very hazy conception of herself. Like very young children who identify themselves with their mothers before developing a sense of self, perhaps she imagines herself as part of a composite entity with Satori, where Satori represents the conscious self and she represents what's hidden. In that sense, her attempts to poison Satori reflect her own attempts to shut her telepathic eye, as she tries to save Satori from the horrors of omniscience. Conversely, Satori's bid to put Koishi to sleep in the second half represent how she's trying to eliminate everything beyond her rationality, and kill all emotion. It's basically what you'd get if both sisters took their respective natures to the extreme, whilst still being driven by a twisted love for each other. Splendid parallels, really. This was a great story.
