Here are some tests done by a friend with his A3-Scout3 build.
325-325 patk (unequipped weapon)
320 atk on lvl 5 twin arrows
0 elemental damage/buffs
target: lvl 2 leaf bug (plate), 3 def
Damage test (small sample):
Reversed damage formula:
Rather than the assumed 200%~ damage multiplier for the skill, it looks like it has a varied multiplier of roughly 315-340%. This is actually insanely strong, considering that Snipe is at 400% (while having much higher base skill atk).
Which brings me to the next point: Twin Arrows may be one of the strongest WB-ing filler skills. 5 overheats, 7s cooldown. Attributes are cheaper as well (compared to higher rank skills). Something like A3-R1-F3 could potentially be the strongest fletcher build with this.