it was 4% boost when i tried it.
also btw, you can just equip 2 of the same element on a 2 hand staff and it will still get the bonus. however, you are wasting 2 slots..
unless there is a way to really improve that boost a whole lot, it doesnt seem like a good idea. a small damage boost is not nearly as good as having resta on the same staff.
i can see this mainly being useful for easy areas where you dont need resta available.
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