Yon can dual class any classes if you are Human of course. Maybe not the monk; bard; Paladin and Barbarian but any other classes with their Kits can be dual class. But you need to have proper stats to make it available. You must have 17 of 18 (not sure) in your strengh if you are a fighter and 17-1
not sure again) in dext if you want to be a thief. The same is true if you want to be a mage (17 or 18 in int) If you don't have high scores, You can't dual class. You need to know what to do in making a character with the idea of dual in the future before playing. Or else, magical items that make higher stats should work.
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Maybe it's because I hate drows more than I hate Humans - Jarlaxle to Entreri