According to the Apocalypse' resistances guide, one has to hit first with the weapon or shield of the betrayer, so that all other damage sources are enhanced by the effect.
GEAR
For effects on gear just add them all up apart from the ones on weapons and shields.
If both weapon and shield (or weapon and weapon) have the same type of effect (like both having absolute reduction) then only the stronger one will apply.
If they have different types of effects then they’ll both work.
You need to hit the opponent with the weapon/shield that has the debuff on it for it to work.
The effect is not shared by the weapons and the same goes for shields
Hence only a defender with his active and passive skills can land the betrayer effect with its shield, as far as i know.