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I completely missed this post. I really know nothing about the "collective unconscious". It's anime so it is possible that they somehow just have a mystical connection with each other and share thoughts. I think they dream up Chiyo chichi because they have both seen the plushies. That doesn't really explain why they both think of it as Chiyo's dad though. Sasaki tells the gang on Chiyo's birthday that the yellow cat is Chiyo's father but I can't remember if this was before or after Osaka dreams of him.