Two weapon fighting takes formal training, which includes systematic pattern development coinciding with drills used to develop the dexterity of the offhand to bring it on par with the dominant hand. The patterns include simple solo drills all the way up to complex two person drills. One of the most influential combat arts in my life was the Filipino systems of Arnis and Kali. (watch some youtube videos on this to get an idea of what I am talking about) But to get back on track, the system stays with light weapons, many of the longest bladed weapons being machete type blades or something closer to a gladius. Being the crazy American that I am, I have used long swords in Arnis practice and it is hard on the forearms and wrists. It takes serious strength, not just dexterity, to wield larger, often unbalanced, weapons in intricate and complex movements.
I haven't decided what the score will have to be, but in my game, I will only allow dual wield styles with two medium weapons if the character has an adequate strength and dexterity skill. A different idea would be to allow two medium weapons but give a penalty to one or both weapons.