Dual-Wielding has been absent from Reach, 4, and 5 with understandable reasons. A good way to include it would be in campaign, you can activate a skull (can be called something stupid like: steel arms, or double chaos or something). This skull would (as you guessed) let players dual-wield certain weapons (or, since this is a chaotic skull designed to be goofy and fun, why not all weapons). The gameplay can be completely balanced for only having one gun in hand, it doesn’t matter. This can be the most overpowered thing in the game, and sort of remain balance (you could get no achievements while using the skull, and X0.00). Also a dual-wielding multiplayer playlist can pop up on certain weekends. With this: campaign achievements still require skill, multiplayer is dual-wield free, and the fans of dual-wielding will be glad.
But take this with a grain of salt, I’m not a game developer so I don’t even know if this is possible.