This is a really fun little series. Koaki should be experiencing a whole heap of existential horror at everything and Chiaki for that matter but it doesn't take itself too seriously so I just rolled with it. The end is pretty bittersweet.
Since I came to see the "sisters" as their own individual persons, I really wanted them to keep up being separate individuals and not overwrite each other. But they really weren't overwritten, they were merged. So they both made it in the end and the omake confirms it.
Fun story, good art, goofy cast, some nice twists and turns and an ending that gets sweeter the more I think about it. Read it.
I put off reading this for quite a while, but now I marathoned it, and I'm quite satisfied. It was thoroughly entertaining, MC was less of a wimp than expected and you could say that best girl won.