What about the Shock Trooper Paragon Path feature?
Deadly Soldier (11th level): When you wield an off-hand weapon, increase the weapon's damage die by one size.
Or the Shocking Skewer power?
Requirement: You must be wielding an off-hand weapon and have a hand free.
With Two-Blade Warrior, "You can wield a one-handed weapon in your off hand as though it were an off-hand weapon."
The Shock Trooper wields an off-hand weapon with an increased damage die. When he wields a one-handed weapon in his off hand, he wields it as though it were an off-hand weapon. If it doesn't increase in die size, he's not wielding it as though it were an off-hand weapon, is he?
-Hyp.