Huh. So you didn't read it...
In ch10 they say that they share a body. And In ch11 they explain that they use concealer and eyeliner to make it seem like they have different placement of beauty marks. In ch11 they also say they didtribute they time equally. So everything so far points to the fact that it is two personalities and not twins.
I didn't read the whole webtoon yet but I randomly checked the later chapters and when their mom shows up she only talks with them separately as they switch and on a photo with her son is only one person.

NO FUCKING WAY! DON'T TELL ME HE'S THE SAME PERSON!!! Either his twin brother died and now hallucinating him thus role playing as his twin since chapter 1! Or he just hallucinated this chapter idk