If you have more than 1 Raven or Hellhound in play, does multiple Storm Spirit or Hellfire stack?
Pretty sure they won’t stack, just like having exact same aura in multiplayer don’t stack together. Unless they change aura mechanic in expansion.
Does anyone know if these auras still don’t stack?
They don’t. Otherwise it makes Conjurers with 2x Briarthorn (and Emboldening Presence) completely broken with 24% extra Physical Resistance.
Working as intended, I think
Imagine multiplayer. 4 shaman summoners (because why not) and everyone has Briarthorn set, which enables to summon two of them. 12 * 8 = 96% physical resistance.
If this worked like that, everyone would run shaman build on multiplayer and be immortal. 
Literally, overcap all of your resists, don’t bother about armor value or absorption and pick one +4% physical resist item/devotion and you’re set. Ravager? More like Pus… 
Even if single player, 24% free physical resist is just… just don’t, please… If this worked, my Ritualist would have 60% physical resist right now, lol. Or 52 if I swapped to full damage weapons.