personaly if i were going to dual class a character id wait until there wasnt really anything else to gain by staying in the original class. If you max out for example a thief, so that there are no more lock picking skills ect., then id dual him to a mage. youll have high skills and better hit points as Bristowe stated. After gaining all the possible skills and weapon proficiencies THEN id dual class him. then he starts out as a mage with decent hit points.
the other thing i supose i should mention is if you take, say a thief, and dual him to mage, make sure he has 18 intelligence. That way you can learn ALL the higher level spells. Spells are limited by the intelligence of the mage. the higherthe intelligence the more fun and effective hell be. I think there is also a limit on how low the intelligence can be in order for you to be able to dual class him. I think its 14. but who wants a mage that cant learn any of the really good spells???? make it 18 and youll enjoy playing him alot more.
IMO a fighter should have a strength of 18 and dextarity of 18 so that hes more effective with missile weapons and has a lower armour class. If said fighter has an 18 strength and 18 dextarity with decent intelligence then hed make a good theif.... or even better....... an assassin. Backstab for lots of damage
Whatever the classes you want to dual from and to, the character should have very high numbers (preferably 18) in the prime attributes of each class.
------------------
THERE CAN BE ONLY ONE!!!!!!!!!!!