I would say you can have many activated at once; the numerical effects may or may not stack depending on how they're described. As for concentration, only if the rule says so.
What do you mean activate your Aura? There's no turning it off... you have it, all the time, no action, just there, always...
Oh, Domain Powers are different. Limited rounds per day it can be active. Standard action to activate, but I am pretty sure you can have as many as you are willing to activate active at any given time. I haven't seen anything like Bardic Performance specifically declaring the previous Aura ends if you activate another.
I may have to check some things, though...
They don’t really stack, but both of them could affect the same targets. Assuming the targets fail both the saves they would be shaken and confused. The only times you may run into problems is if the effect contradict each other. Paladin’s have multiple auras and all of them affect those within their range, why would clerics be any different. In fact the word Aura is more fluff then game mechanics.