From Halopedia, the Halo wiki
Double Grenade: Charge, R, pause,L, L
When you release a plasma bolt after the pistol is charged or when another plasma weapon has overheated (Beam Rifle, Plasma Rifle) wait one second and press L, L in order to throw two grenades rapidly after each other. With the right timing you can throw them almost at the same time. You can also set traps with this, deliberately missing a plasma shot, then surprising your opponents with a rain of grenades. You can switch back to your secondary weapon, but only after the first grenade is thrown.