Stras is wise. Heed him :)
Also, I'd posit for a fictional effect rather than a mechanical bonus every single time! A compendium 'master swordsman' that specialises in wreaking mayhem with their bladed weapons? Freaking awesome. A move that grants me the fictional power to injure them where they are vunerable instead of a boring +1 damage? Priceless.
Following from that, a thing I threw together:
When you've spent time, money, and blood learning to fight well with two blades, you can take the move "Hidden Threat".
Hidden Threat: When you hack and slash with a single blade, hold 1 for the rest of the battle. When you draw your second blade and reveal that you were just holding back this whole time, you can spend 1 hold when you do damage to do +1-damage.
Once you've taken "Hidden Threat", you can take the rest of these moves whenever you level up, in place of your class advancements.
Cunning Strike: When you can bring both of your blades to bear, you can spend 1 hold from "Hidden Threat" to injure your enemy where they're most vulnerable (the GM will tell you how).
Sever: When you can bring both of your blades to bear, you can spend 2 hold from "Hidden Threat" to sever an opponent's limb.
Parry: When you counter an attack using your off-hand weapon, spend 1 hold from "Hidden Threat" to avoid damage from that attack.
Counter (Requires: Parry): When you parry an attack, you can spend an additional hold from "Hidden Threat" to inflict your damage on the attacker.
Serpent's Fang Strike: When you pounce on an enemy with both blades drawn, spend 3 hold from "Hidden Threat" to close the distance to them and inflict your damage.