Split Shot / Splitting Steel interaction

The description of the two is Split on Impact respective on target.
I assume this means the same.
Do they overwrite each other, or is it additive?

And how does it interact with piercing?
Do they split again on the next target/impact?

Since projectiles split on impact it does increase boss damage too, at least with Nimiz, right?

Could I calculate this in PoB, by adding 2 projectiles for an average of 2 assumed piercings * (2 splits * 50% chance)?