I am looking for a book I read when I was younger. It involves 2 boys from different world swapping places and pretending to be each other. They are identical except that one is white and the other is red, and when they swap places, they change colour except for the area around the groin/arse area. This stays the colour of their home world. The boy from this world also had been speaking what he thought was a made up language for years before he even met his "twin", and it turns out it's the language from the other world. There is also a scene where in the other world he discovers the sun actually travels through a tunnel in the earth when it sets, taking it back to where it can rise again. At some point they discover that their fathers, also identical, had at some point done the same as them.