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It's a simple technique where two shuriken, like the Fūma Shuriken, are piled one onto another and thrown simultaneously. However, depending on the way it's used, it can prove its absolute efficiency. The trick is to somehow draw the enemy's attention towards the upper shuriken. Then, they have to deal with the path of the lower, unnoticed shuriken. However, if the enemy notices both shuriken, the technique ends up losing all of its efficiency.
Ikari usually uses regular shuriken instead of Fūma Shuriken.