Ok, there’s two different rules that sometimes get confused here.
Radiation damage is a damage type that reduces maximum HP.
The Radioactive Effect is an effect that allows other weapons to inflict a little Radiation damage as well.
So, with the example above, you’d inflict 2 Physical damage (the damage from the attack) and then each Effect would also inflict 1 Radiation damage (from the Radiation effect), reducing Max HP by 2 overall.