Okay so easiest way to differentiate the twins is the type of piercing they have. Seongwon only wears studs and you can sometimes see him not wearing them at all (like in the earlier chapters), while Yeongwon has multiple piercings such as helix piercings. You can also tell by their personalities. Seongwon is a bit serious, introverted and tends to keep to himself while Yeongwon is more extroverted, smiles more around other people (not exclusively like Seongwon does when it's just with the mc) and is overall just a social person.

I’m having a little trouble differentiating them? I think I like seowon more but I honestly can’t tell which one he is