I guess that you didn't didn't understood the question properly (or just used improper words). Double handed swords and other weapons already are in game. He wants to have equipped two meele weapons at once, but as I understand not necessarily fight with both at once.
It isnt such hard, long ago I've done it by dynamically transforming one of swords into sword-like crossbow and enabling special mds which changed its sounds, animations and fight mode to that of sword. The only problem was with killing uncoscius enemies, game was always crashing when i tried to do that.
Jestem nawet prawie pewien że gdzieś te skrypty opublikowałem, prawdopodobnie w podobnym temacie w problematyce. Stanowią zarazem fundament do robienia duali w których nie musi występować podział na lewo- i praworęczne miecze.
There is although one else problem. As I remember, when data of the weapon is changed (for examppe, its type) it's changed for all current and future instances in the game. I guess that npc's who are already spawned and equipped won't have problems with using such weapon, but in case of these spawned or equipped after change, there may occur Unforseen Consequences.