As far as I understood, Yuni was framed as a character who enjoys cheating on her girlfriend because it's thrilling in her otherwise boring dating life.
Fuuko was framed as a character enjoying the thrill of stealing someone else's girlfriend.
They don't really love each other, they only love themselves and their own satisfaction.
Neither Yuni nor Fuuko actually want to be in a serious relationship with each other ("I'm fine being the second girl", "I don't even like her"). So, to put them both in a serious relationship is self-defeating, because all the thrill will be gone.
So, I'm not sure how Nanase can ever satisfy Yuni now that she tasted the forbidden fruit. She's been corrupted by the poisonous Fuuko as the several mentions of the fungus suggest.
So, Yuni will probably go back to cheating.