Stacking would be when you have two auras of the same, such as a Dream Helmet and a Dream Shield. Both grant a Lvl 15 Holy Shock aura, and if they stack, they would then net me a Lvl 30 Holy Shock aura.

So basically, its adding like aura levels up.

With a Lvl 20 Lightning Mastery, the Lightning damage bonuses of a high Holy Shock aura are insane.


However, you can't do this on a merc now, and the same may or may not be true for your actual character.