The Person Controller by David Baddiel

Fred and Ellie are twins. But not identical (because that's impossible for a boy and a girl). They do like all the same things, though. Especially video games. Which they are very good at. They aren't that good, however, at much else - like, for eaxmple, football, or dealing with the school bullies. 
 
Then they meet the Mystery Man, who sends them a video-game controller, which doesn't look like any other controller they've ever seen. And it doesn't control any of their usual games. When the twins find out what it does control, though, it seems like the answer to all their problems. And the key to all their wildest dreams. At least it seems like that...