If anyone's interested, I got around to test this, and they stack indeed. I gave both items to Pallegina (I figured it would be harder to notice on a character with Carnage) and she was hitting twice on retaliation, whereas, obviously, armour without shield would hit just once.
I'm using Tidefall, so no shield for me, but I suppose this means that the damage output would be increased with it and/or other retaliation items (I don't know if there are others).