The dual-wielding rogue is a common trope in fantasy games like D&D and Dragon Age, but was it ever considered a viable combat style on a larger scale? To clarify, I'm not talking about a sword and shield, but rather carrying a weapon in either hand and using both of them for offense and defense. I suspect that the complexity of learning to fight with both your dominant and non-dominant hand would make learning this skill quite difficult, and thus would be rare in military combat. So, what's the deal?
Generally speaking, no.
Now that I've made a broad declarative statement I'm going to immediately undermine it by saying there were a few historical examples. Some cases in Japan of wielding a full length sword in one hand and a shorter one in the other for example. Some cases of vikings supposedly using combinations of weapons and shortswords. The one thing you'll almost always find in such cases was that they happened not by choice, but because a shield was in some way impractical. Japan didn't really make shields for example. Raiders expecting to meet light resistance might forego the extra weight, the better to carry more things off. Stuff like that.
As a choice, when there was an option available, the only reason not to carry a shield was if a large two handed weapon that gave you extra reach was available. Fighting with two weapons requires a great deal of space, impossible in a tightly packed formation, a great deal of time and effort spent training, impractical in any large scale army, and at the end left you in a contest of skill that you might very well lose anyway. Especially when a proper shield makes a useful offensive weapon in its own right, the practice is only feasible if you can ensure you'll never have to fight someone with a shield.
While I am unsure as to their use on the battlefield, dual weapons (being two weapons of the same shape and size) are found in the treatises of Renaissance fencing masters from Italy.
The 1536 treatise of Achille Marozzo has a section on the use of two swords. His own words state, "I will compose for you a play of case of swords that makes an excellent affair,' which means he viewed the use of two swords highly.
Marozzo's illustration of a position with two swords.
Thirty-four years later, we see the 1570 treatise of Giacomo di Grassi. In the beginning of his section on dual wielding, he states, "...they be not used in the wars." This is just talking about the use of dual side swords, so it doesn't rule out the possibility for dual wielding other weapons to have happened on the battlefield. He also says this: "To him that would handle these weapons, it is necessary that he can aswell manage the left hand as the right." He confirms your suspicion that the swordsman would need to be trained in the use of both his hands.
EDIT: Forgot to include Di Grassi's illustration of a position with two swords.
To answer specifically this part of your question:
was it ever considered a viable combat style on a larger scale
The answer, with only perhaps a few exceptions, and then those would be arguable at best, is: no. Large-scale combat throughout almost all of human history prior to breech-loading rifles, focused on formations of men facing off against each other. While there were sometimes irregular formations of skirmishers or other groups intentionally meant to break up formations, melee fighters were almost always presented in formations of one kind or another.
Fighting with weapons in either hand is totally impractical for fighting in formation, because it takes up a large amount of room. You would be in danger of hitting the man next to you with your off hand, and he with his, unless your movements were exactly coordinated, which would make you predictable. As a result, these kinds of setups were never used on a larger scale -- though they may have been used for dueling and other small-scale combat.
Now, I presume you're only talking about melee weapons, but if you're getting into the era of early firearms, say the 17th century, then it was by no means uncommon for sailors boarding a ship in combat to fight with a cutlass in one hand and a pistol or dagger (which would often be thrown) in the other. Since there would be no formations here, in most cases, and a man might find himself one against two or three in some cases, it is possible that he could give himself an advantage by having multiple weapons to his opponents' one.
what about pistol cavalry in the 1500's and 1600's. I had read about carasoul formations where riders would hold a pistol in each hand, and when the got to the front they would fire one, turn, fire the other, then report to the rear to reload.
