Synch Blade

Synch Blade is an ability which allows a Keyblade wielder to summon another Keyblade during battle. So far, the only characters to use this are Roxas, who can use this ability to summon his two Keyblades, the Oathkeeper & Oblivion, and Sora while in Valor Form, Master Form, and Final Form.

Roxas can use Synch Blade because he has both Sora's Keyblade and Ventus's Keyblade, 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 come back together, Sora is able to mimic Roxas's Synch Blade within several of his Drive Forms, but since Sora possesses every ability that Roxas had, it is also possible for Sora to use Synch Blade naturally, without going into a Drive Form.

Kingdom Hearts II and Kingdom Hearts II Final Mix

 * Roxas learns Synch Blade immediately before fighting Axel in the Basement Hall. The ability cannot be used after the fight ends.
 * Valor Form has Synch Blade as a default ability.
 * Master Form has Synch Blade as a default ability.
 * Final Form has Synch Blade as a default ability.

Kingdom Hearts 358/2 Days
While Synch Blade is not an ability outright, Roxas can wield two Keyblades in Story Mode on Day 358, and in Mission Mode by equipping Zero Gear with three Ability Units.