Technically, they're not "stacking", either. In your example, the PC would take 10 Necrotic Damage from the Zone, and 5 Nec. Damage from the Aura- separately.
If you had Resist 5 Necrotic, you'd take 5 damage from the Zone, and no damage from the Aura, for example.
So, from this reading, I would have to say that DracoSauve's reading is wrong. Regardless of damage type, same monster or different, you only take damage from the most damaging aura. Thats how I interpret anyways.
Again, zone has updated wording as well in the MM2. The last paragraph is virtually identical to aura paragraph listed above.
Do you live in the South Pole? 
