Yeah, any Human character can dual class, but like Entreri said, you need to know what you want to Dual into BEFORE you create your character. For example, say you have a fighter, and you have given him/her top points in strength, constitution and dexterity, but rubbish points in intellect, there is no point in trying Dual to a Mage. Also, you will lose your Fighter bladed and piercing weapon skills. KNOW WHAT YOU WANT!
I like to start with a fighter and Dual to a Cleric, so I give her max Strength, max Wisdom, as high as I can on Constitution and Dexterity, and not less than 11 on the rest. Which means you need a MINIMUM Abilities total of 90.
Also, I only allocate weapons skills to the Fighter that the Cleric can use. When you Dual, you will LOSE the Fighter skills, and start Cleric at level 1. However, if you Dual at level 7, by the time you get out of the first dungeon, you will be lvl 7 Cleric - and when you are lvl 8, you will get your Figher skills back (though not all the Quick weapon slots). Have fun!
F.