[Faceman]

the basic idea in dual-classing is to gain basic abilities in one class before specializing on your real class. a standard tactic is to start as a fighter to get better THAC0 and more weapon proficiencies and then dual to mage, cleric, thief or another more versatile class.
dualing a thief can make a great fighter with limited backstab abilities (full backstab if you dual late enough). However, I would not recommend to dual a mage for example, as this will leave you as a figther, thief or whatever you dualed to with some magic missiles and maybe Web and Acid Arrows.