Rulebook, p. 165
"Unless noted otherwise, if the effect of a special rule in some way alters a characteristic, the result of a dice roll, or any other value (such as a model’s armour value), that effect is cumulative. This means that the effects of duplicate versions of the same rule combine together, increasing its effect.
For example, if a model is under the effects of a spell that grants it Armour Bane (2) and carries a weapon that has Armour Bane (1), the model would be considered to have Armour Bane (3)."
Impact hits are neither a modifier, or a value - it’s an amount of impact hits, thus it doesn’t stack.
On a different note: I just won bronze in a local 10-man tournament! Report and learnings here