That would cause it to be too over-powered. I myself would vouch for it have double to near-double the damage, but half the attack speed. That way it won't be pushing down those whom have single sabers.

In a typical melee spar scenario, it would go like this. The person with double sabers would have to attack once, then back up and circle for another chance to maximize the usage of the doubled damage and minimize the penalty of slower attack speeds. Whereas, the single sabered player would be able to land 2 hits, 3 if they're lucky, before backing up due to the fact the double sabered player can attack. In reality this is perfectly balanced, and would require knowledge, skill and comfort with your weapon of choice for you to come out on top.