Whirl Wind: Robert Liparulo

This was book 5 of the series. David and Xander discover they can go from 1 world to another by following portals without going back to the house. They go to the Civil War world and get the doctor they were asked to get the first time and in doing so rewrite history. Evidently the war is shortened by saving a wounded man. They then go into a portal taking them to a torture chamber, and then to an ancient world where battles are fought on elephants. They make it home alive, but then are taking to Atlantis and sold into slavery before escaping and making it back to the house,