From Halopedia, the Halo wiki
Dual Melee is a glitch that happens in Halo 2.
In Halo 2, when wielding a Plasma pistol, press the X and B buttons repeatedly. This will cause the Plasma pistol to produce its normal melee animation, where the Master Chief uses his fist, and then another where he hits with the gun itself. This can be hard to see at first, since the animations occasionally "go through" one another, but it is still there.