I don't remember :-(

what do you mean by "split up"? like when they were in high school and the misunderstanding happened and Ritsu went abroad? I can't think of any other split up but if you've read the entire manga then you know that there was never an entire chapter about that, it was mentioned since the first few chapters then they had flashbacks to before the split up.

Yes, that split up, when they were younger. I never get to know what was the misunderstanding. I re read it, but I still don't know what happened
I thought that maybe it was included in an extra, or something like that...

oh is that so? Because for me, I watched the anime then read the manga (from the beginning) and because the misunderstanding was mentioned in the very first couple of episodes in the anime, I didn't notice that it wasn't in the manga. It has been years since I read the first chapters so I'll pick it up again, if I happen to read the chapter you are asking about I'll come back to this thread and tell you. Meanwhile, if you're super curious about the split up then I recommend you watch the first few anime episodes

This misunderstanding was revealed in volume 1, chapter 1. XD

Or on here, it's this one:
http://www.mangago.zone/read-manga/sekaiichi_hatsukoi/mh/v01/c002/1/?av=2

Basically, the misunderstanding was because Takano laughed when Ritsu was trying to re-confirm that they were dating. This caused Ritsu to conclude that Takano was only playing with him and not taking his confession seriously.

That misunderstanding is cleared up in the present, but now Ritsu is still debating with himself whether to go out with Takano or not because the misunderstanding practically crushed him mentally, emotionally that he wants to avoid that kind of hurt.

Thank you!! (づ￣ ³￣)づ

So... the misunderstanding was the confession (Ritsu), the laught (Takano), the kick (Ritsu) and the disappearing (Ritsu)... （￣へ￣）
I always thought there were more in the story, something more dramatic, I don't know.

I end up re reading it again...ლ(´ڡ`ლ)