Note that the empty hand in "single-handed" weapon is used all the time - to punch or grapple or throw things or whatever. So it's not like mideval folks didn't use the off hand for stuff.
That being said, I'd model any sort of off-hand weapon use as a flat +1 bonus to defense (for a sturdy parrying dagger), and to use Brawling (which covers the use of the dagger) when someone tries to grapple you and you have to use that dagger offensively.
For wielding equally-long weapons, I'd personally rule it as a separate Ability based on Single Weapon, but with a flat +2 to defense. Here's my reasoning.
(Martial art sword geek digression time!)
Dual wielding is something of a misnomer: a spear and shield grog is wielding something in both hands, and if he slams you in the face with his shield before stabbing you, it's certainly going to feel like both were weapons. as you mentioned, if you're going into combat, your best bet (as a melee-based foot soldier) cross-culture, is almost universally a spear + shield, with some sort of sword for your general-purpose back-up weapon. However, both spear and shield are obviously pieces of military equipment, and crazy-insane annoying to carry around with you every day, so folks usually developed other things if they wanted to kill each other off the field of battle.
So you end up with civilian styles where you may or may not be allowed to carry at best some sort of civilian weapon (such as a german messer - literally a 'knife', by mideval german law, but a 'sword without a pommel' to anyone else). later on, rapiers were similarly civilian weapons designed (out of the evolution of the longsword) to deal with light-to-no armored foes in the streets of Verona and whatnot. And these folks definitely chose to carry things in their off-hands to increase their chances of survival - the main gauche and buckers are both common (and relatively small) things you can carry around with you on a regular basis along with your sword. Or alternately you can just use your cloak, which is actually makes for quite a good defensive block/entanglement/light armor for your off-hand. (swords aren't lightsabers - you can grapple them through heavy cloth without cutting yourself. obviously you can't block a full-on swing with your forearm, but for indirect parries it works just fine.)
However, you may note that there really wasn't much use for carrying around two full broadswords or two rapiers to use simultaneously. There are descriptions of folks doing so in the middle ages and beyond - but almost always in the context of sport competitions or something similar, where a master wanted to try something difficult: the so-called "brace of rapier" style of fighting.
The main issues are as follows:
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It's hard to carry two full-sized swords around with you. In a civilian context, ease-of-carry and quick-draw are more important that cutting power, which is why no one really carried two-handed swords on a regular basis for duelling.
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It's hard to quick draw two swords at the same time, which is context in which civilians usually end up fighting.
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Even if you did get them both out, the one in your rear hand will always be six inches to a foot shorter than the one in your lead - just because almost every trained melee fighter stands with one side that much closer to the opponent. To counter that you'd have to stand completely square to your opponent - and in doing so, you'd present a much larger target for them to swing at, and you end up being relatively flat-footed. So it's not all that great to strike with.
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Finally, having an empty hand in an urban brawl is usually more useful than having another full-length weapon.
So most folks are left with "the long and the short" - usually having a smaller, defensive tool in the rear hand that doesn't NEED the range of the lead hand weapon. This has the advantage of being significantly easier to carry (and put away when not needed) , and more suited to the reality of civilian urban combat.
If you do attempt to dual wield, say, sabers, the reality is that the sword in the rear is still almost always out of range of your foe, who is basing HIS strike distancing off of the lead weapon. You do have the advantage of being able to lead-switch faster (ie, attack in both the right lead and the left lead). You also end up with a long, thin, metal shield to hide behind. So you end up with lots of blocks and counterstrikes that you wouldn't otherwise be able to do - which, again, is probably justified just fine with a flat +2 defensive bonus.
Striking, however, isn't significantly faster - you strike primarily with your hips and your legs (like a boxer), and that power is transmitted just fine with a standard fore-hand and back-hand strike with a single, one-handed weapon. In contrast - holding another weapon in your off hand won't make your hips turn any faster. That being said, you can go for a bit more speed - but you end up loosing a lot of the power of half the strikes that you would otherwise have. So from a game mechanic standpoint, it's probably a wash to model that aspect of it.
So... yeah. +1 defensive bonus for having something in your off-hand you can deflect blows with, or a separate Ability with a +2 defensive bonus if you want to wield two equal-length weapons. It's very much related to Single Weapon, but Ars Magica doesn't do "defaults to another skill" very well - it's pretty much the equivalent to a D&D Feat. Alternately, call it a minor virtue that can be learned over the course of a season, and be done with it.
EDIT II - as OneShot mentioned - I'd say just treat the parrying dagger as a 'regular' shield that a One Handed Weapon Ability would reasonably have been trained with - but dual-wielding is arguably a different Ability, albeit one closely related to One Handed.
EDIT - for an interesting discussion of historical mideval swordsmanship, I'd recommend scholagladatoria, on YouTube - he's a british historian-type person who is involved in HEMA (Historical European Martial Arts), and he puts out a lot of short videos about the techniques and texts of mideval fighting. Here's his discussion on dual-wielding: http://www.youtube.com/results?search_query=scholagladiatoria+dual+wielding