I know I already posted not so long ago, so... uh, sorry?
Anyway, the book is called “The Tragic Tale of the Celestial Lily and Crystal Silver”
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Excerpt:
Crystal stares at the newcomer. No one has defied her before- surely this is not her author’s doing? She is perfect and the other characters are nothing compared to her, so how dare this-!!!

The Celestial Lily- or Lily Silversmith- stares at the queen. Her demon deal should have warped everyone’s feelings for her, right? So why…?
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Crystal is a fourth wall-breaking Mary Sue who has a superiority complex because she has her author’s favor, and Lily is a “Mary Sue” who made a deal with a demon to be super powerful and loved and all that, and together they must... um, defeat the demon that Crystal unleashed to make her deal, I guess.
Basically two characters with superiority complexes and deep-rooted insecurities about the fact that they didn’t earn their positions clash while everyone else assumes they’re friends, because they’re both “perfect”. So the fate of the world should definitely not be in their hands, but that”s okay, because according to Crystal, it’s a book, and the heroes always win in books!
I also have no idea how collaboration is going to actually work so there’s that.