Synch Blade is an ability which allows a Keyblade wielder to summon another Keyblade during battle. So far, only Roxas is able to use this ability to summon his two Keyblades naturally, the Oblivion and the Oathkeeper in battle, and Sora while in Valor Form, Master Form, and Final Form.
Tetsuya Nomura stated that Roxas's ability to use two Keyblades implies that Sora can use Synch Blade without the Drive Forms, due to the fact that Sora has every ability that Roxas has. This is proven when Sora takes Riku's Way to the Dawn during Xemnas's final battle with them. While handling Way to the Dawn and his Keyblade, Sora does not handle the Keyblades like Roxas or his other Forms do, Way to the Dawn is held upside down and his Keyblade is held normally, he also has many attacks that he uses that he has not used before in any Form including Standard Form. It is also shown in the Xemnas battle that when using the beam of light that acts as an attack and is used to open any Gate or lock that when Sora does it with Synch Blade he still uses only one Keyblade to do it.
Spoiler warning: Plot and/or ending details follow. (Skip section) The reason Sora holds Way to Dawn upside down may be a reference to how Ventus holds his keyblade. Spoilers end here
