I bookmarked some chapters lol:
(I could be wrong and left out some parts. These are the chapters where they’re seen together, I didn’t include chapters when their characters appeared separately. If you wanna know the full backstory/context, you’d have to read the first season.) 
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Chapters 16, 21, actually starting something in chapter 25-28, 30-31, 33-36, 42-46, 48 and 49 onwards.
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If you want a summary of their characters without reading the first season fully:
Eunho (top): toxic fckboy that the 1st mc had a crush on (Eunho is two-faced and is like mc’s dream guy). Eunho doesn’t like sleeping with virgins, so he only agrees to do it with mc when mc loses his virginity. He’s only interested in sleeping with mc and finds him entertaining.
Rakhyun (bottom): toxic ex that stalks the mc, wants to be with mc cause he’s the first dude/person he got an erection for? (So he’s a virgin and once he experienced getting an erection with mc for the 1st time after his “trauma”, he was obsessed to be with mc to fix his ED).                                            

		
Love the second couple way more than the first hansol really pmo in the first chapter crossing boundaries and ect and they’re lowkey bland