Dual Melee is a button combo 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 John-117 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.
It is unknown if this is a hidden animation left by Bungie.
If done using a Sangheili character model, the Elite's mandibles will open completely. You can see this in The Codex and This Spartan Life machinimas; the actors use the glitch for theatrical reasons.