I’ve read all nine and I can’t say that I noticed any real differences between the two.
Each chapter in the books is written from a specific character’s perspective. From what I understand, each of the characters was written consistently by one author or the other. So, for instance, all the Naomi chapters would be written by the same person. They may not have stuck 100% to that, but I think that’s how they tried to fit things together.
https://en.wikipedia.org/wiki/Widdershins
Just because it sounds cool.