Synch Blade (シンクロブレード, Shinkuro Burēdo?, lit. "Synchro Blade") is an ability that appears in Kingdom Hearts II. It allows the user to summon a second Keyblade during battle. So far, the only characters to use this are Roxas, who can use this ability to wield his twin Keyblades Oathkeeper & Oblivion, and Sora while in Valor Form, Master Form, and Final Form as well as Double Form.
Roxas can use two Keyblades at once because he has both Sora and Ventus's Keyblades, due to Ventus's heart remaining within him. Originally, he just wields Sora's Keyblade, as Xion's creation had siphoned away Ventus's Keyblade. However, when she is defeated, his powers return to him, and he reawakens the ability to use Ventus's Keyblade. After Sora and Roxas reunite, Sora is able to use Synch Blade within Valor, Master, and Final Form, but since Sora possesses every ability that Roxas had,[?] it is theoretically possible for Sora to use Synch Blade naturally, without going into a Drive Form.
In Kingdom Hearts II, Synch Blade is a support ability. It grants the user access to the abilities of both of their weapons.
An interesting note is in Kingdom Hearts II, Roxas dual-wields with Oathkeeper and Oblivion, both of whose abilities are related to Drive Forms, which is needed for Sora to access Synch Blade.
Learning Synch Blade
Kingdom Hearts II and Kingdom Hearts II Final Mix
Kingdom Hearts 358/2 Days
Kingdom Hearts III
Kingdom Hearts III Re Mind
Notes and references