The Double Advanced Rasengan, is simply two Advanced Rasengan used simultaneously. Once created, both Rasengans are used at the same time with greater force than a single, regular Advanced Rasengan. It is different than Twin Rasengan, because Twin Rasengan uses one Rasengan at a time to attack, while this technique uses both Rasengan at the same time. This, of course, requires quite a bit of chakra, likely double the amount used in a single Advanced Rasengan, which implies it might require Advanced Uchiha. As a result of using both this technique and the Advanced Rasenshuriken, Ryun used up all his chakra, forcing him out of Advanced Uchiha.