Alright, I've gotten this request thousands of times, so I thought I'd clear up my thoughts on the matter and ask for your feedback (assuming you'll read this and think a bit before posting)
First of all, I've thought about dual-wielding in great detail since the humble beginnings of the mod. I knew about the Battle Gear mod (which is crazy legit) before it was famous, but I'm not going to make Hack/Mine compatible for a number of reasons. However, Battle Gear has laid out a few awesome ideas I wouldn't mind, um, borrowing (like holding any two items at once, not just weapons), giving credit where its due, of course :)
There are four main aspects of having two, functional hands to consider:
There are no issues with wielding two-handed weapons. Easy peesy. Functionally and balance-wise, they'd be no different than before.
Dual Wielding Weapons
Dual wielding weapons presents a big problem... how should weapon swings be handled?
The realistic and therefore Minecraft-y approach is to give each hand its own timer. You'd either give the user two attack buttons, let them swing the left hand only if the right hand is unavailable, or simply have them alternate hands with each click.
The issue I have with this is the click frequency-- if you have a fast attack speed, it's already annoying to spam the mouse, and with two weapons, you'd have to spam it twice as fast! Worse yet, if its a different button for each hand... well, we'd be imposing a worse definition of "skill" on the player than previously defined.
The only viable way to have it on seperate timers, using the same timings, is to have weapon swings be semi-automatic, repeating when the button is held. This ability could be the default for all weapons, or it could be class specific, or even applicable after a certain attack speed threshold. But then, you'd be attacking so quickly and easily, that no monster could touch you! Well, possibly. This is how I felt in vanilla Minecraft, when you could just spam the mouse to swing your weapon without limits. This could easily become incredibly broken (more broken than the Mage's ability to exploit towers for levels by spamming lightning while standing outside of them.)
So what if we tinker in different ways? Have one timer shared by both weapons, which would visually alternate, but would effectively deal the damage of both weapons with each swing. It wouldn't be as "realistic" and therefore Minecraft-y, but it would give the same effect of holding two weapons, with approximately the same DPS. We could get further disconnected from reality by applying a damage reducing multiplier to damage output, in an attempt at balance (under the premise that your swings are inherently weaker with two weapons, much like WoW), but I'm not sure how the players would feel about this. Maybe it'd be acceptable, but who knows.
Shields, shields, shields. There's a lot of ways to handle this, so many that I'm not sure I could summarize my thoughts on them. You could have it semi-Battle Gear style, where you have a shield-exhaustion timer, and by raising the shield, you gain a major defense bonus while depleting your exhaustion. This could be amazing for boss fights and monsters with certain attack patterns. At the same time, it'd be difficult to balance, and easily broken. Alternatively, you could simply give the shield a defense bonus plus whatever magical properties are on it, but this seems boring to me, given the possibilities. I'm wondering if there's an entirely different way to handle it, but I'll leave you to decide.
Dual Wielding Items
This isn't necessary, but would be a great little feature to add to the mod. Definitely a clever invention by the maker of Battle Gear (dual wielding and the like being intuitive), and I'd definitely be giving him credit if I were to add it. I can't think of any immediate problems, besides how best to assign hotkeys. It may not be a greatest use of my time, though :P
So basically, I'm undecided, but this being Minecraft, I'm leaning towards the more "realistic" options. Things like holding the mouse to raise your shield (and maybe even moving the mouse to reposition it), or clicking for each individual weapon swing, add a lot of charm to the game that I'd hate to deprive the player of. I may be able to forge balanced ways for each of these aspects while maintaining realism, but it's hard to say. Would the less realistic ways of handling dual-wielding be acceptable to you? Gimme yo' thoughts!