Responses
No, they interacted multiple times before chapter 31. Just look for the cues where the MC starts to over think about their relationship. Usually the other twin is the cause of that, so he's in the chapter right before. Or you can look for the chapters where the fake ML starts to act distant or refined, that's when the fake ML appears.
Jesus! This is really serious! Poor danwoo
woaah...
So is this the first time they actually interacted? Like real contact? Or some other time in the past when pretending to be hajun his brother talked to danwoo?