From Halopedia, the Halo wiki
It is possible to be combined with the Double Grenade.
First, you stand on a dual wieldable weapon, holding a dual wieldable weapon, then press L to throw your first grenade, hold Y to pick up the dual wieldable weapon, B to melee and throw the weapon away and L to grenade again. With default settings, the button combo should be L,Y,B,L.
Note: This trick can also be used in combination with the Double Grenade, using the plasma pistol as the first dual wieldable weapon for a deathtrap of three grenades.
List of Button Combos