Favored Augmentation (2nd Level):
Select one system of the body for which you can acquire augmentations. When you purchase and install an augmentation with an item level no greater than your character level into that system, you reduce the augmentation’s price by 50%.
Do individual arms count as separate systems for the purpose of this ability, or one system? I'm inclined to think that they count as separate (and you would have to choose "left arm" or "lower right arm"), but would like some clarification about the intent of this rule.
Given that they do less damage than normal weapons and that TWF isn't nearly as powerful as it was in PF, it doesn't seem game breaking for a player to get 2 boneblades for the price of one (one in either arm) if both arms would benefit from this feature, but it might also be too good for what you give up and, at any rate, not what the developers intended (I'm big on the latter).