I read this book in the 90's. As I recall it centered around brother-sister twin assassins; after the brother's body was taken over or killed, the sister scooped up the brother's essence and they "live" together in her mind as she searches for a way to get her brother's body back.
I thought it may have been a Mercedes Lackey title, as I remember the book's cover being in her style, but I can't seem to find it as one of her books. The background world had some fantasy elements but I don't remember much. I remember a passage in the book about the hero being frustrated that her broken wrist would never be as strong as an unbroken wrist. Any thoughts?