Skull splitter and A thousand cuts are both talents that shoehorns the player into specific weapon loadouts, on a class that is all about weapon diversity. Slayer wields two melee weapons, and I think diversity in what you want to bring should be encouraged as it is a core concept of this career.

I suggest that both talents should be able to work regardless of loadout, but should only apply when a two handed or one handed weapon respectively is currently in use.

So if I use Skull splitter and bring the two handed hammer and the dual hammers I would get the bonus power while swinging the two handed hammer, but not while using the dual ones.

While this could be interpreted as a buff, it is also true that we are willingly bringing a weapon that gains no advantages from the chosen talent. I see this as a way to increase diversity in weapon loadouts and builds without making Slayer too powerfull.