The way I see it, dual wielding could easily fit in with crpg... coming from the realism angle, using only one sword and having a free off-hand was more effective in real combat because you used that arm for grappling maneuvers, which aren't present in crpg. Also, dual wielding was used in the far east (very rarely, like pretty much only musashi and his disciples) and let us not forget that for better or for worse we DO have eastern weapons present in crpg. You can't say that it would only fit in a renaissance-era setting while it was indeed around at this time, just not in the west. Also looking at crpg now.. there are plenty of unrealistic things going on... archers, xbows, and throwers (was throwing even a dedicated "class?" I always thought that warriors like vikings would occasionally throw their axes if necessary... but seriously... someone who ONLY threw shit? wtf...remove throwing) who fight openly in a battlefield with no sidearm. They belong on top of a castle's walls! Also melee heroes without a shield... no one would be yoloing around like that in real life unless they were a fully plated knight... and even then many knights used shields. Sure, there are stories of warriors going berserk and wielding a giant 2handed axe and wrecking shit, but there are just as many stories of vikings picking up second swords/axes and doing similarly heroic feats. Long story short, crpg isn't at all a medieval combat simulator... its just a game about swords.
I think that if dual wielding was ever implemented in this game (or another similar one) it should be similar to real life, where dual wielding was harder to use, and gave very little payoff. To preserve the "difficulty" of using two weapons I would suggest that you swing the left-hand weapon by clicking the LMB and moving the mouse in the desired direction, to swing the right-hand weapon you click the RMB and move the mouse, and to block you click in the scroll wheel (MB3) and move the mouse in the desired direction. Adding the 3rd button kind of, if only barely, makes it more difficult for players to use. Also, the only advantage a player gains from using this method (in crpg or warband at least) would be that they could wield two different weapons at once, therefore having two different option to attack with... maybe a stabby sword and cutting axe, or a mace with knockdown and a high-damage cutting weapon in the other hand... or the ultimate tincan-cracking combination of mace + pick.
I personally would love to see dual wielding implemented just to play around with all the endless combinations... short spammy wep + long slow wep... stabby sword + cutting sword... the list goes on and on. Granted, I would much rather have existing "issues" worked on long before this would ever be looked at, it would still be a neat feature. But of course even if the devs thought it was realistic enough to add, the main challenge is the fact that it would be one heck of a bitch to code and make the custom animations for... so as Paul said we will never see it in crpg. Would be cool, won't happen.