Synch Blade is an ability which allows a Keyblade wielder to summon another Keyblade during battle. It is unknown if in the Kingdom Hearts series, there are any characters that are able to equip this ability. 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' ability to use two Keyblades implies that Sora can use Synch Blade without the Drive Forms, due to the fact that Roxas has every ability that Sora has.