Synch Blade

Synch Blade (シンクロブレード) 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 summon his two Keyblades, the Oathkeeper & Oblivion, and Sora while in Valor Form, Master Form, and Final Form.

Story
Roxas can use two Keyblades at once 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 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.

Mechanics
In Kingdom Hearts II, Synch Blade is a. 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 the Oathkeeper and Oblivion, both of whose abilities are related to Drive Forms, which is needed for Sora to access Synch Blade.

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.

Kingdom Hearts III
While Synch Blade is not an ability outright, Double Form allows Sora to wield two Keyblades. Roxas also continues to wield Oathkeeper & Oblivion as a party member in the Keyblade Graveyard.