Curate, connect, and discover
YQY is the only one that knows and that's because he has a situationship with SJ. And to the others it just looks like Shen decided to start pulling his punches and are waiting for the other shoe to drop.
If you go BingQiu route he finds out by walking in on a phone call about how frustrating Liu is, the guy not the gal, and is like, 'Wait this ISN'T the guy who makes him miserable!?!?!?'
All the main characters live in one building that Shen Jiu's an administrator/landord of. Everyone hates him except for Yue Qingyuan. (I gotta figure out a backstory for them).
One day, something happens and Shen Jiu have to go somewhere and asks his twin/younger brother Shen Yuan for help - he would live for a few months in the building and act as the landlord. Shen Yuan agrees.
It's not intentional for Shen Yuan to impersonate Shen Jiu - it just happens when everybody mistakes him for his brother and he's now kinda stuck with it since he's too awkward to correct them now!