Poisoned Spirit + Chilled to the Bone

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I am experimenting with the combination of Haunt (Poisoned Spirit) which causes enemies to take 20% more damage from all sources and Summon Zombie Dogs (Chilled to the Bone) which also causes enemies to take 15% more damage from all sources.
If they don't stack, I'm guessing which ever is highest takes effect, meaning Chilled to the Bone is not useful when working with Poisoned Spirit, but if they do stack, is it stacked multiplicative (23%) or additive (35%)?
What is or how would I calculate the outcome of these two runes?
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Poison - Until You Suffer Some (Fire \u0026 Ice)
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