Reading Response to Scarlet

I am reading Scarlet by Marissa Myer and I am half way done with it. One thing I have noticed about Scarlet is how it ties in with Cinder. It does switch between the two characters, Cinder and Scarlet but I know they will meet up soon. As I was reading, I could tell they would meet up because Cinder says she will go to Paris and that is where Scarlet is. I like how all the books in this series will connect like this.
I wonder why the author made it to so that the characters would connect like this. Scarlet and Cinder are very different characters not only physical but also how they act emotionally. One thing that surprised me was when Scarlet defended her grandmother so quickly, you can tell she really cares about her. What I like most about this series is that the books are based off of fairy tales. Cinder is based off of Cinderella and Scarlet is based off of Little Red Riding Hood.