I wasn't able to find a definative answer in the spell descriptions. So I'm hoping to find the final answer to this question.
Can you stack aura spells?
I have had numerous people tell you can. There is a high number of people I see casting a 15% damage and then casting a 20% damage right after. They seem to be convinced they are stacking damage.
Do aura spells cancel each other out if they are different in their effects, Example: can a conviction spell be cast on top of a vengence spell and have both in effect? Can two damage or accuracy spells be stacked?
If you refer to the the update history for Oct 2010 when Celestia and Astral schools were released under the Star school info is states that Auras DO NOT stack. Hope that helps.