Dual wielding doesn't seem to fit here.
1) They would either have to make a gun that is two weapons. Thus making it to where you couldn't select the guns you were dual wielding. I believe it would be a special weapon.
2) If you are talking melee wise. I'm sure that can be added as a subclass, but they would have to make it different then Bladedance. To keep all subclasses one of a kind.
3) If they let it be to where you could use any weapon to dual wield it would most likley not be allow in PvP. Due to it throwing off the balance of all the weapons.
4) Ok, now, I understand that Bungie made a Halo. I get that and Halo is an amazing series. But Destiny isn't Halo. If they wanted to just add all the Halo weapons and guns in a "MMO" type setting they would have.
5) Also, you do realize that if they use something from another game. That can end into a lawsuit or something like that.