In truth Tommycat from a historic point of view a person using a single handed weapon such as a Longsword or Bastard Sword without a shield actually used it less aggressively and more defensively. Meanwhile when using 2 swords or a large double ended weapon as would be conceived in a double bladed sword those styles of fighting were much more aggressive.
Your standard 2 weapon fighting was not a defensive fighting technique your intent was to do harm to your opponent or multiple opponents at one time and the typical fighter he wielded 2 weapons generally charged into battle both arms swinging wildly in an up and down chopping motion moving quickly with his slices and generally overpowered his opponents.
As compared to your single weapon style of a single blade most typically found with Bastard sword but also longer Long Swords. There are specific styles of combat for using these weapons they easily enough the High guard and the Low guard. The High Guard you held your sword high usually at chest height or inverted at a downward angle over your head. Low guard you held it defensively from your waist. Both were typically used depending on your opponent and what his stance was to best defend and counter your opponent.
I'm going to curse myself but I'm going to point to the Death Knight Class in World of Warcraft. This class has the ability to tank in fact is the only plate wearing class that cannot use a shield which makes it a good example. Now in WOW for many reasons you don't duel wield to tank you use a 2-hander there are mechanics as to why such as having reduced chances to block with the 2nd weapon unless you have an exceptionally high hit, also better stat bonuses overall in the two-handed weapons.
While I use the example of from WOW above it is not the only example of games and game systems where duel wielding weapons for defensive purposes is considered either a waste in the long run or a bad idea. Usually you have to put more effort and training into the ability and sacrifice other abilities in the long run that are much more advantageous.
Working so, we will learn history as a tree knows it; we will climb into shapes printed in the seed; we will become time made visible, years made fragrant; we will make of concentric memory a stem of praise; we will inhabit daylight at a trees own speed; we will be travelers who remain, patriots to this ground.
OMG I only rank 62.13018% geek or a Extreme Geek as it is referred to on the geektest at Geektest