I got two Storm Rulers in a single NG.
I walked into Yhorm's boss fight and Siegward immediately appeared with one of these in hand. I assumed Siegward was the one supposed to kill the boss so I just rolled around and watched onion bro kill the giant by himself. After the fight I was automatically teleported to another boss.
When I came back to Yhorm's room I looted the Storm Ruler in front of the throne and then I got a second one along with Siegward's loot.