Don't bother with dual casting. It seriously isn't worth it.
It might be better to ignore anyone who mentions not bringing a companion. Yes, they are a pain. Yes, they get in your way occasionally. However if you play an unarmored mage you will be made of paper and will die a lot, especially if your opponents get close (and they will). A melee companion like Lydia will help keep you alive a bit longer.
My mage had Lydia through level 38 and she was awesome.
She didn't allways do much damage but she did take the agression off of me and my atronachs/reanimates.
However I do dissagree with dual casting.
For most spell schools it isn't worth it,but for conjuration and reanimating it is.
Giving your reanimates X2 duration is great because you can't simply conjure another like you can with atronachs.
Illusion might be another school of magick that can benefit from dual casting,but I'm not sure.
My last mage didn't use Illusion at all so I can't say for sure.