Spartan182 discusses in his blog the possibility of plasma energy swords being dual-wieldable in Halo 3, based on suggestions in the Brute video documentary.

One might think this would make such characters a bit too powerful, although I'm not really sure how carrying two instant-kill weapons really makes a difference unless you can lunge in two directions, but perhaps I just lack imagination. He points out that being able to hold a sword and another weapon simultaneously-- that might be very useful indeed.