Certain thing's stack. For instance if you had Champion Point's in Armour penetration and the sharpened trait to reduce more Armour on your weapon's... those stack.

If you are trying to reduce your target's Armour with ability's, those do not stack unless one is Minor and one is Major. Two Major's will not stack (same with Minor's).

You can get the information here.

http://esoacademy.com/buffs-debuffs/

@sneakymitchell

Hey, if you have any question's you can just send me a message through the inbox. I don't mind helping out when i can. I like to think i know a thing or two about all the mechanic's etc.