The Enchantress by Michael Scott

The two that are one must become the one that is all. One to save the world, one to destroy it.
   Sophie and Josh Newman traveled ten thousand years into the past to Danu Talis when they followed Dr. John Dee and Virginia Dare. And it’s on this legendary island that the battle for the world begins and ends.
   Scathach, Prometheus, Palamedes, Shakespeare, Saint-Germain, and Joan of Arc are also on the island. And no one is sure what—or who—the twins will be fighting for.
  Today the battle for Danu Talis will be won or lost.
  But will the twins of legend stand together?
  Or will they stand apart—one to save the world and one to destroy it.