Doppelganger

Union Magic: Doppelganger (合体魔法『』) is a shadow and darkness-based Union Magic spell.

Description
After slipping into a shadow, the Dark Magic user and the Shadow Magic user emerge from another shadow behind an opponent and simultaneously attack, with the Shadow Magic user mirroring the attack of the Dark Magic user. The effectiveness of the copied attack is unknown as it is stopped before causing any damage.

Trivia

 * Doppelgänger is German for "double-walker" and refers to a duplicate of another person.