Those 2-Star EE Scrolls.

I was wondering, is there a limit like only being able to use these on an equip once? Because the scroll itself doesn't state anything about the % rate lowering, or does it do this and just doesn't say it does?

The scroll will only work on the item if it has either no enhancements or only one; using the scroll will bring the enhancements to 2 stars, after 2 stars has been reached then it can no longer be enhanced with them. The success rate remains the same for both no enhancements and 1 enhancement.