They were born at the same time. One lived in the USA and the other in Germany. Each would have "episodes" where one would pass out and then would see through the awake one's eyes. This is how they grew up, until one day the boy was kidnapped and she had to go save him. He was born into a royal or noble family. The story is then about her trying to save him before it was too late. The world is otherwise normal and there are no other powers or psychic people that are described in the book.
I read it in 1996 in my high school library.