Dak And Stell/The Travelers Trilogies

Two boys live on a planet that has magic thick in the air. So thick that they can shape it and when they pass certain tests they get a tome to copy the spells into so they can use the magic without shaping, but when the boys go to each receive a tome nothing happens and then a single book appears! Except it isn't a book, it is a god that lost a bet and they are transported to another world to try and save a world that is on track for an event that will end all life on that planet.