I played as a sort of jack of all trades my first time around, but i wanted somthing a little different this time- i wanted to be a pure mage. I never used duel casting before, as i always had my longsword in my primary hand, using spells only to heal myself and provide a little extra damage.

Duel wielding magic gives no advantage whatsoever. I do exactly the same amount of damage, yet spells cost significantly more- AND it takes up another hand. Whats up with that?

strange i am in a dungeon with skeletons , a single handed fireball wont kill one , a double handed fireball will . bug?

There are a couple of perks that makes dual wielding magic better. Doubles the damage, makes the target stagger etc.