In the book The Host, is Melanie and Wanda different people?

Ok, so I have read the book. But Im kinda confused. Okay, so I know that "Wanda" is in Melanie's body right? So their the same person... right? But when I was on youtube.com I typed in "The Host Cast", just to see who people thought the characters would look like. But in ever video they had a brown haired girl for Melanie, and a different blonde headed girl for Wanda. Now maybe I missed something in the book but I thought they were they same person. I could be wrong. Could someone please explain this to me? Thanks :)