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In the Harry Potter and the Deathy Hallows, Harry used three wands to perform the Stupefy Charm to Fenrir Greyback, who was lifted off his feet by the triple spell, flew up to the ceiling and smashed the ground.
Having said this... using more than wand is ABSOLUTELY a quite huge advantage for wizards and witches to win a battle or a duel. If someone tries to cast a spell on someone, particularly the stunning spell, the one that receives the attack will surely fly high up into the air.
The question is, if every wizard in the books wants to win every single battle blocking their way, why don't they use more wands? That will surely help them a lot!