my mercenary(defiance) have 21 thorns and 15 conviction auras...do i get the same levels of them when im near him.or a little less ? should i be the one whos carrying that item to get to full advantage of it
The 2 auras are different with regards to what they affect.
The thorns aura is an aura that's 'on' you, your merc, summons and/or party. So yes, if you're within the auras range of your mercenary you'll have the thorns aura 'on' you and return physical melee damage done to you.
The conviction is different, it does not affect you. It affects monsters and other players (if they're hostile) around you - or in this case, around your merc. You'll still benefit from the monsters lower resistances and defense.
Btw. - Level 15 conviction? Are you sure it's not level 12?