Firstly, there is no such thing as a boss weapon in Bloodborne, ind if there was, it'd definitely be the Burial blade, not the damn Rakuyo.
Thirdly, how does it have more health? Maybe your weapons are stronger, but go back and test it, please.
Additionally, when you're trying to fight both at once, it's like trying to get out of a electrified penguin while you're being pelted with flaming bats with chainsaws all while you're having a seizure from the fact that you're actually meant to be dealing damage to them. The actual way to deal with ***** me and ***** me harder is you're meant to go down, kill the winter lanterns roaming the tunnels (they give you sweet *** blood gems too) and farm them from the little semicircle cave from above. You can get leap attacks in, but they'll have a job of it hitting you. You can even knock the second one down early.