The Rise of Karzoug
Rule: Any feat that may be applied to a single weapon may, instead, be applied to a weapon pair. For example, instead of Weapon Focus: Longsword, Valeros might choose Weapon Focus: Longsword & Shortsword. If so, Valeros would get no bonus wielding a longsword alone, no bonus wielding a shortsword alone, but would get the +1 to hit when wielding a longsword in one hand and shortsword in the other.
In the same way a pair of weapons may be enchanted to function as a pair. Instead of a +1 longsword one could enchant a set consisting of one longsword and one shortsword to +1 for the same cost. Again, use individually the weapons would have no bonus, any only gain the +1 when used as a pair.
Design Note: This rule is intended to reduce the gap in effectiveness between two-weapon fighting and two-handed fighting and make dual wielding more attractive. As is, a two weapon fighter pays twice as much for his weapons than a two-handed fighter, is strongly encouraged to wield the same weapon in each hand, and under-performs the alternatives when not making a full attack . These have the unintended effect of meaning that the fighting style of Valeros, the iconic fighter, will never be played by anyone familiar with the system. I’m not too worried about increasing the power of two weapon fighting as an option as it is almost never seen in comparison to great weapon fighting.
Realistically training in paired weapons has historical precedent in things like the samurai daisho and thematically the idea of weapons enchanted as sets is appealing to me.