one of the first home brew rules we established was duel wielding incurs no penalties.
used logic... i trained from young age to use swords and the like why wouldnt i have trained to use either hand? what if something happened to my main hand? wouldnt either i or whoever tought me have thought of this? im an elf ive lived for two hundred years surely i would have picked up this ability?
More used logic... RL knights did train from a young age to use a sword, and almost without exception they trained only to use the sword with their primary hand. Significantly more in the medieval ages fought left-handed, as the culture of forcing left-handers out of the habit had not yet been established.
Why didn't they train ambidexterously? Because they knew from experience that it tended to be a losing game. If you had lost your main hand, common sense dictated that you run or surrender, since that kind of injury mean you would shortly succumb anyway if you didn't stop to do first aid (or the medieval equivalent).

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.