I think everyone knows this one, but for those that don't:
This move requires that you have nades. Either kind, doesnt matter. To pull off the double-melee move, melee once, then immediately throw a nade, then immediately melee again. If you connect on both melee's, you're opponent should be almost dead, in about the same amount of time it normally takes to melee once.
Note: This move can be hazardous at times if you use a frag nade, as the nade you throw may hit your opponent and ricochet (sp?) back and hit you!