Bold Stance is effectively the Dual System of Awakening, where both the lead and support unit would attack unlike the Attack Stance of Fates, which requires an unpaired unit to attack (the supporting partner may be another unpaired unit, or the lead unit in a paired set).
Unlike the Dual System of Awakening, where the support unit would randomly block attacks, the supporting unit of the Bold Stance may only block attacks when the shield bar for Guard Stance is filled. Negating enemy dual attacks this way will still deplete the shield bar. Dual attacks from the user's partner and the enemy's partner in Attack Stance will also increase the guard gauge by 2 when they attack.
Bold Stance is enemy-exclusive, and cannot be learned by playable characters.