Item Break Formulas, Two Swords and You

[Archael]

As we all know, Break skills will fire twice if the user has Two Swords.

In fact, I think they are the ONLY ability which is boosted by Two Swords.

What is responsible for this?

The skill slot in which Item Breaks are located?

or the Item Break formula?

Any info is appreciated. This is for the Arbalist job.