If for a tempest you take the baseline of a scourge which is off hand, does 1d8 damage and is +2 to hit, then you make it a superior version giving it +3 to hit you almost have the double sword (in stat terms)
I would say that the addition of the defensive property is what makes it slightly overpowered. That and the fact its cheaper to enchant than two weapons. If it was up to me I would drop defensive and charge double for the items
Does a scourge count as both a heavy and light blade? It's that + defensive that is so good. Plus the savings on getting two weapons of course.