cinderhulk and sunfire passives should stack together

I think that since cinderhulk and sunfire are not the same items, they should have different unique passives. I sometimes want to buy sunfire on top of cinderhulk but it's kind pointless. After i get a randuins and i am looking for another hp-armor item, the only choice is sunfire but then again the passive won't stack with cinderhulk, which makes me nit want to buy it