I'm not much of a mage player. My Cultist is my recent character that uses destruction magic and the like but I heard something interesting. Let me get this straight. If I cast 100% weakness to Magicka 100% weakness to Fire Magicka (for example) in one spell and it lasts 10 seconds, then I hit an enemy with it again it stacks? I always assumed it'd simply reset the duration.
Say you have a spell:
Weakness to Fire 100% for 10 seconds on Touch
Cast this spell, then within ten seconds, cast your fire damage spell, and do double damage.
Here's an interesting scenario:
Say you have a spell:
Weakness to Fire 100% for 10 seconds on Touch, and you named it, "Fire Prep 1."
Say you have the exact same spell, with a different name, "Fire Prep 2."
Cast Fire Prep 1
Cast Fire Prep 2
Cast Fire Damage Spell
Do quadruple damage.
Marvel at one-hit kills that doth flow from your fingertips. Well, not exactly one-hit, as you are using two prep spells, but... you get the idea. Touch is faster, and almost a guaranteed hit, than target. Also costs less, magicka and money-wise.
EDIT: And yes, put the Weakness to Magic on the end of each prep spell. Might want to add soul trap to the damage spell (depending on how killer the spell is) seeing as to how much damage you are dealing.