With Form "Water" you can use any style, double bladed, single, two lightsabers. As long as you stick to the form. It is very difficult to master and even Jaden has not mastered it. He is the only known practicier though. The explanation for Form "Water" is as follows: "Empty your mind, be formless, shapeless — like water. If you put water into a cup, it becomes the cup. You put water into a bottle it becomes the bottle. You put it in a teapot it becomes the teapot. Now, water can flow or it can crash. Be water my friend."