Hello all,

I started a new character interested in obtaining all the daedric artifacts and I was wondering whether you can put a second enchantment on the items when you reach 100 enchant and the 100 enchant perk. I was interested in adding elemental damage to one of the weapons.

Nope, you can't. You can only put two enchants on an item when you first enchant it, so you can't add on stuff you have enchanted with 1 effect either.
One of the enchanting skillbooks explains why that is so.