Two-weapon fighting styles evolved because people didn't have shields (civilians), or don't like carrying shields (they are cumbersome), or wanted to be flashy. That's pretty much it. A shield is better both for offensive and defensive purposes than an offhand weapon.
Indeed. In fact a lot of the off-hand dagger techniques are similar to using a buckler (or small shield), if not derived from those techniques. Going larger than a small shield probably has different techniques, though, more-so based on large unit formations (various shield wall formations).
Also, soldiers fought in much larger units than a fantasy adventuring party, and also adventuring puts you in a variety of environments (cities, jungles, dungeons) that a soldier wouldn't face on an open field, so cumbersomeness can be a factor. Your characters in such a game are more like a mongrel of soldiers and civilians and egocentric heroes that just want to look flashy. More like gladiators, I guess.
Anyways you're right, DOS is not a game that tries to be realistic.
Though, how easy to put in dual wielding is an issue, especially if we want it to be mechanically distinct instead of just ore damage with less accuracy). And if dual wielding and unarmed is put in, I'd again like advantages to using a weapon with a free hand.
Common or effective is another matter, but there are historical manuals showing 2 rapiers being used.
But how many pages of such a manual would depict this? I'm willing to bet it's like 2 pages, with the rest of the manual describing more useful techniques
