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My first thought was Combination Attack, but a Combination Attack is usually a little more involved.
Like, if they are working in perfectly coordinated tandem to strike the opponent several times and end it with a huge simultaneous punch/kick (kind of like Kyosuke's team up attack in Rival Schools
, or Roberto's, or several others'), I think that counts. But just walking up and punching together once at the same time is a bit of a stretch or a Downplayed Trope at most.
Or if it's a special punch, like they both shout "Flaming Fist!" and it's a double extra supercharged punch compared to if they punched solo.
Edited by KD
I was thinking of something similar to the effect of the final mirrored kicks in Roberto's example.
Two allies punch an opponent at the same time in the same area(usually the stomach) in a mirroring pose for a combo attack.
Always a dramatic finisher and sends the opponent flying.