It really depends on which way your going about it, if you mean to dual class as in Mage and Knight/warrior or you mean the advanced class Warrior-mage.
But, as a basic outline.. I'm also making the assumption here that your planning on Mage first, adding Knight/warrior at level 5
Level 1-4 - Mage
Start off as a spell slinger, with Class A Armor and Weapons. You get all the whizzy spells for a Magex1 and can dip into those lovely enchantments that everyone is going on about, although you'll have natural access to them, so it might not be worth getting enchants depending on how you view your character. Probably focusing a bit on Int as the main stat, and maybe some Stamina to cast the spells in combat.
Level 5 - Choose your Physical Class. For the sake of this I'm going to say Knight, but both they and Warriors have the same rough advancement.
You gasp.. I get class D Weapons and Armo..oh.. wait.. dual class. Rules are.. Best weapons.. worst Armor.. so your shiny new mage with a hint of knight to them can suddenly, strength allowing, pick up a buster sword, but can't put on plate mail.. so basically, for taking a physical class, at the moment, you get access to better weapons, you suddenly have a realization that you need to perhaps point points into other stats like Strength and Resistance and maybe Life as well, rather than just Int and Stamina.. and the potential that if you did get any enchants for spells rather than casting them normally, you might not be able to use those enchants for a while.
You'd still be able to grow your own natural spells, which would be the way to go for it, rather than enchants at the moment.. but basically you'd be a mage who walks around with a big weapon.. and having to balance out your stats rather than min/maxing yourself to be a glass cannon or whatever style of mage you want.
Its a viable choice although you just need to work around the understanding that your never going to be able to truly stand toe to toe with a pure warrior of the same rough level, or a mage.. at least on their terms, but you'd have a toe in each of the physical and magickal camps to give a little bit of fluidity around things. You'd have to balance your stats and your spells, perhaps sinking large amounts of coins into focus enchants to reduce the stamina cost of your casting, and you'll have to be picky on your armor, as some doesn't allow for spell casting, or can be difficult in other ways.
You'll perhaps max out with a phys def of 25 or so, which when you see people talking about their 40 or more, might make you think..
.. but thats the choice you get for going down a hybrid path as it were, you give up somethings to play something that you want.
If you, at level 10 or higher, take mage into an advanced mage class then currently, to the best of my understanding at this point in time, you'll get access to Class B armor, which might or might not do any better for you, plus you'd have to pick and choose on your spells. Each Elemental mage loses access to the opposing elemental spells.. so you go firemage.. you lose Ice shards, chill touch for example. You do get the choice to allign to one of the Maigkcal Towers and access their unique spells, or remain unaligned and have a limited spell choice.. so thats something to consider in your choice of things.. but you'd still get your big sword/weapon of choice to walk around with.
I'd take some time looking at the options.. Hybrid classes can be fun, can hold their own in most places, although will never be top of the class in the majority of situations. But if your planning on that route.. start planning your advance class now, just to make sure that you don't pluck spells that your going to lose in that transition.
Of course, you could always evolve the physical class into something else.. Mist Raider, Paladin.. or even Warrior-Mage itself.. and if you get there.. and realize you've made a bit of a mess.. then you can always reset and rebuild, a limited number of times without penalty, to find something that fits you better.
I'd say like at the players around, see whose got characters in that style that your looking at.. and ask if you can talk to them and explore their thoughts on it and the like and see what they think, what issue's they've found and suchness.
And to that end.. as I've got a Magex5/Paladin.. feel free to poke me if you want to ask questions on things about how I've worked with the foibles of such a hybrid.. or post em up here and I'll respond for all to see.