Books are sorted by the "back-drop" of the story. Meaning if it's a crossover, whatever the main "world" is that's what category it's under. 

(EX: TVD x PJ where a PJ character is living in Mystic Falls and going through the storyline of TVD is sorted into TVD no matter how much of the PJ plot is there) 

If the plot lines are even then I picked one category or another at random. Didn't want to put them into multiple to save space. 

Not that anyone asked...