Nagatsuki already knows that Chiyo and Ookuma had a thing going back in Middle School. That's what Chapter 170: Story of the Runaway Man, was all about. Of course Nagatsuki has yet to confront Chiyo as to why she, Chiyo, never told her, Nagatsuki, that she had been interested in Ookuma and what a jerk he actually turned out to be. Frankly, I think Coco must have early onset Alzheimer's since Nagatsuki was shown to have eavesdropped on Chiyo and Ookuma's conversation back in chapter 71.
As for future chapters, I don't discuss them here, even in spoilers, though you're welcome to PM me if you wanna talk about them.
I think its pretty clear that the Coco is painting a failing relationship. As to the “why don’t they talk” comments, well most people are pretty crap at communication. Its got to the “lets bitch at each other” stage which is a pretty bad sign.