You missed the key point of my post:
Ambidexity is a born ability. Ask any physician. You cannot learn true ambidexity as what the PHB and BG series describes!That said, you can learn to use two weapons with the same hand with ease. I'm not saying the inital benifit of giving rangers better use at dual wielding isn't unfair or without merit, but giving them imporved useage and above and beyond what a fighter could ever hope to achieve is. A same fighter who decided to train with 2 swords, lets say broad swords, (because both are usuable by the same class and both are considered weapons of size category to aquire the penalties mentioned). With practice and training if a fighter had the ability could do just as well as the best ranger.
If you want, have ambidecity a chosen ability that must be picked first for fighters before they get
IA, but they have the same potential, if not moreso because they, unlike their ranger counterpart, don't haveto spend their time memorizing spells, learning about nature, etc. Here a fighter should ALWAYS outclass a ranger. Fighting is what a fighter does and a dual wielding fighter and vs a dual wielding ranger using only their swords and all other things being equal should always win over the ranger, except maybe if they're really low level because the ranger's special training he had gotten, but a fighter, if you devote him to dual wielding should ALWAYS be able to suprass the best ranger in all pure fighting techniques and potential in every manner. The ranger has skills that help augment this, that is why he's a ranger and not a fighter.
Still on that term they are warriors and along with monks and barbarians should be able to get 3 stars in a weapon prof.
Also as to thieves dual weilding, that was mostly with daggers or short swords, although the penalty doesn't (or shouldn't) apply if the offhand weapon is either reguardless of the size of the other (although i'd haveto say trying to do so with a claymore or no-dachi should give them penalties the same, but as they can't be done in
BG2, it doesn't matter).