I like the enchanting system in skyrim, mostly. The enchants are fairly balanced and while switching between various sets is inefficient, it's not unduly extraneous. The one aspect of enchanting I take issue with, however, is that I can't add a second enchant to quest rewards, random loot, etc that only has one enchant; assuming I have the Extra Effect addon at this point. A key example would be the helm of nightingale armor. I don't cast illusion spells, so it was useless.

Long story short, I'm looking for a mod that allows me to add a second effect to armor even if it has one enchant already, once I do have the aforementioned perk. If anyone can help point me in the right direction, that'd be fantastic!

there isn't one.
but you could get an enchantment cleanser mod and then just re-enchant the item with 2 effects you want.

http://skyrim.nexusmods.com/mods/9894