Resistant Skin cannot stack since it only sets the unit's magic resistance to a hero's (immune to charm, reduced duration of spells etc.)
Hardened Skin does not stack to my knowledge, though it could be stacked via items (different cooldown groups perhaps?) or by putting it in a spellbook, then putting that spell book in a spell book (repeated as many times as you need hardened skin to stack) - but that's messy.
Those are the only methods I know of, but there should be another way.