In the 2nd edition rules the off hand was mandated to be smaller in size and weight than the main hand, thus it allowed the rapier and main-gauche combination but not a pair of rapiers. The penalty was -2/-4 reduced by the dexterity reaction adjustment. So in 2nd edition you couldn't dual wield longswords.
In Baldur's Gate 1 they used the optional weapon proficiencies, which set dual wielding penalties higher to begin with and had it reduced by levels. This means they didn't follow the 2nd edition ruleset for it.
In Neverwinter Nights 1 they began at -6/-10 and allowed you to dual wield longswords, but the penalties came down to 0/-2 provided you took two weapon fighting and ambidexterity feats and the off hand was light.
3rd edition was the same as NWN1 in terms of how they handled it.
Neverwinter Nights 2, however, baked ambidexterity into two weapon fighting giving you a free feat essentially.
So in both previous editions of dnd, two weapon fighting, or dual wielding as it can be known, has changed so many times not just between editions but also rulesets for the video games using them, that it's definitely best to simply learn how the game itself handles it and not to worry bout what has gone on before.
