Is there a story about the other couple shown?

I was wondering the same... they seem like an established couple. I looked around on the net, I wasn't able to find any of the authors works that look like it might be about Ichitaka and Rito. :/
If you have better luck finding it, share? :}

I have a feeling that in later chapter there will be more about the second couple.

Exactly! This book has three stories:
1) Ore no Shiranai Murakami-kun.
[Part 2] Ore Shika Shiranai Murakami-kun.
[Extra; this comes after the two stories below tho] Kazoku no Shiranai Minowa-kun.
2) Suso kara Rouraku Erotic (The story with Ichitaka and Rito)
3) Ore no Twins!
You can see the couples in the cover actually :3
http://www.mangago.zone/read-manga/ore_no_shiranai_murakami_kun/sa/sa-c001/3/
manga/ore_no_shiranai_murakami_kun/sa/sa-c001/4/