That's an interesting thought. I also wondered if people would like it more that way, but I personally feel that it loses its charm once it's split into two series. Because then, it just because two regular BL manhwas, isn't it? The plot twist is what throws you off because most people have a presumption when reading stories that the main character HAS to be there, HAS to be seen till the end, and we let that presumption make us take for granted stories that have a more complex narrative structure---simply because it's "easier" to read regular, linear stories.
True, but plot twist only leaves an impression for a bit, then you get used to it and it's no longer a twist, but rather a constant occurrence, and for slice-of-life type of stories, there is not a lot of shocking developments, so it comes down to whether you love all of the characters present. When you only like some of them, then having to put up with the other characters for, say, ten chapters in a row, or rather, not being able to read about the characters you like for ten chapters, make you feel quite unsatisfied.
However, splitting the story into two means that to get the most out of the story, you would still need to read both stories, so if you didn't like one of the two couples, you maybe missing out on some of the events of the story simply because your favorite couple wasn't involved. But you probably won't care much about their story anyways so you're not missing much.
A really ambitious alternative would be to split the story into 4 or 5 stories, with each of the main characters as the protagonist, and retell the story in each one's perspective. There will be a lot of overlap in events, but at least we get to learn more about what each person sees and how each person thinks. However, it puts a lot of strain on the author to keep multiple stories' timelines concurrent and consistent and to redraw same scenes from multiple angles, and to keep each story a little different from the rest, at the same time, to keep each story interesting by itself. It would take quite an effort for this to work and there are a lot of ways for it to fail...
if the story was split into two series, one focusing on Kyubin and Seungtaek and the other focusing on Seunghee, so people who are interested in only one of the couples don't have to read through all these chapters about the other couple, which they may find boring. It also means that the author could describe events involving both couples from multiple perspectives without making it feel redundant and achronological, which the story sometimes feels like.