????

So I read a book this summer about twins that were seperated; one was believed to be dead but the other knew that wasn't the case/The Children had been taken to live on spaceships?? Away from their families; that lived in the "underside" of the world. There was a giant wall surrounding the world and this was a great secret b/c most of the people believed that the world had been destroyed and could only live inside this wall. These twins were meant to save the world essentially. Okay - super rambling but I hope that this will help me to find the title (seeing as how a 2 hour Google/Amazon/Everything search has proved completely un-useful)