Dont know much about prop design, but a quick search shows that it uses a hollow shaft/tube for one prop with a solid shaft inside of it for the second prop. Some type of gearing is needed between the two shafts in order to get them both to rotate in opposite directions. I think the USS Albacore used that kind of design
here is a diagram showing the props
http://www.freepatentsonline.com/6702631-0-large.jpg