Yeah, they didn't really dual wield back then, but they did often use two guns. Who cares about realim? They already threw that out with the reload times and accuracy. Dual wielding would be fun for pistols and swords. I don't know why anyone wouldn't want dual wielding.
For the extra realism to keep them immersed in to the game. Some people are like that, if it's not realistic then they feel like they're playing a game, when they're trying to get immersed in to the game and feel like they ARE there. --- Tell me my grammar/spelling sucks, I care... really.
It's cute how everybody keeps talking about 'realism' in a fantasy game where we're using magic.
Suspending the irony of that argument however, those saying that dual-wielding single-fire, muzzle-loading weapons isn't historically accurate, are wrong as well. Based upon the images available so far (I'm not going to bother finding one, the gun you unlock in Pub Games is a prime example), the guns seem to most closely resemble traditional flintlocks.
Flintlocks came in many designs and, as everybody's mentioned, had horrible reload times. However it's because of this reload time that 'dual-wielding' was favorable. With only one gun, you still seldom had time to fire and reload before firing again. Because of this, many designs came out that had multiple barrels that could be loaded beforehand. (A fun example: http://en.wikipedia.org/wiki/Ribauldequin )
However, these designs never caught on because of their higher price and more frequent problems (misfires, muzzles overheating, powder in multiple barrels going off at once...). In turn, it was more efficient and reliable to carry multiple single shot firearms.
Scenario: Pirates boarding a merchant ship. Men on either/both sides firing their pistols as quickly as they can before dropping them and drawing their cutlasses. Or, a pirate alternative that became viable as flintlocks became smaller (and that pre-dates the creativity of Square), a flintlock/dagger combination which allowed the wielder to fire a shot and then continue to use the pistol as a dagger should close-quarter combat occur before reloading is viable. (Figures that trying to google an example is bringing up mostly shopping results for replicas, here's a replica for example anyway: http://www.amazon.com/1800s-Flintlock-Pistol-Dagger-Combination/dp/B000NKF782?ie=UTF8&s=sporting-goods&qid=1205235219&sr=8-2 )
Other common scenarios of dual-wielding that would be more realistic but that we're less likely to see in Fable would be the use of a pistol in one hand and a cutlass in the other. Or the other end of the spectrum. Rather than using the pistol first and then a melee weapon upon closing, using a rifle at range and then falling back to a pistol as a last ditch defense at closer range.
Some quick links for those interested in flintlocks: How flintlocks work: http://science.howstuffworks.com/flintlock.htm A brief overview (with a touch of humor) of how flintlocks affected history: http://www.strongblade.com/history/flintlocks.html And, of course, the catch-all, wikipedia: http://en.wikipedia.org/wiki/Flintlock