When dual wielding, it is best to think about things before you just grab a needler and a plasma pistol, and have a pistol as your third weapon. You should make it so you have a good backup weapon when dual wielding, ammunition can go quite quickly. Have two plasma rifles (or your choice) and have something like a particle beam rifle, shotgun, Covenant carbine, or perhaps a rocket launcher so that you have something to fall back on. Some suggestions are as follows:
Needler x2
Plasma rifles x2
SMG x2
SMG and a plasma rifle
SMG and a plasma pistol
Note: Having two plasma pistols is not a good choice because you cannot hold the two triggers down, or you will power them up. You must keep pulling, making this somewhat difficult. When wielding two weapons at once, use only one at a time, and when that runs out of ammunition, use the other one while reloading. Do this to keep up a steady stream of fire.
Dual needlers is probably the most underestimated weapon pairings available in the game, and probably the most powerful as well. Needlers have an incredibly fast rate of fire, are heat seeking, do initial damage then explosive damage, and you can pick up usually 80 to 160 ammo after each bout of gun fighting (in campaign mode). The only bad thing about needlers is the fact that they ricochet off shields and some body armors (for example, Hunters and the Elite Guard). They are not recommended for fighting shield wielding enemies, Hunters, Elite Guard, or Infected Flood (as they are extremely quick and will most likely do damage to you before the needler shots explode in them). In multi-player mode with friends, there should be nothing more feared then dual needlers.