The book was a children’s/YA book published sometime between 2005 and 2012. It was meant to be a quartet. I know 2 came out and I was curious if it was ever finished.

It had two main characters, a boy on earth who was disabled because he born on a world with magic. And a girl who is from earth but was raised in the world with magic, she’s the only one who can’t use magic and is immune to it.

There are evil magicians who can travel between the worlds, but doing so damages the worlds. The two main characters can move back and forth together but it damages the worlds. They’re told the only solution is to seek help from each of four elementals, societies of magic creatures that, if all four can be convinced to help, can make a bridge between the worlds in a safe way. Both of the main characters have to go to their birth worlds or it’s bad for the fabric of the universe or something, and this can only be accomplished with their help. The bad guys use dark magic and want to hunt them down. The boy struggles to learn the magic of his birth world. The first book dealt with the water elementals and the second with the earth elementals, I think. I think the name of the magic world started with an F

There’s a scene where the girl is on earth for the first time while they’re on the run and they go to a McDonald’s and she experiences a soda for the first time. I think her name starts with a K (Kira?)and the boy’s might be Marcus.