Combiner: I take it humans cant dual class? What're the pros and cons of multiclassing and dualclassing?
On the contrary, only humans can dual class and only non-humans can multiclass.
For example if you make an Elf Fighter/Mage, you start as Fighter level 1/Mage level 1 and the XP you gain is divided to both your classes. Both classes are active all the time and raise in level up until the XP cap. In TOB you will be able to chose from both classes' pool of High Level Abilities.
On the other hand you can start as a human Berserker (single class chars can chose a kit, in this example Berserker is a fighter kit). You start as a level 1 fighter and raise him to level 9. At this point you decide to dual him to mage (second class can't chose a kit). From now on your abilities as a fighter get deactivated and you start as a level 1 mage. You only keep your hitpoints intact. Saves, thac0 etc are of a level 1 mage. This is usually called the downtime of a dual. You raise as a single class mage up until level 10 (one higher than the deactivated class) and then your first class gets reactivated. You gain all the benefits if better than the current ones (thac0, saves, attacks per round, weapon proficiencies etc). From that point on you are a Fighter(Berserker) level 9/Mage level X (all XP you get goes on your mage levels only, fighter level is set).
Few additional notes. In order to dual class you must pick a combination that is a valid multiclass, eg no Ranger/Mage or Sorcerer/whatever. Furthermore you must have 15 in your primary ability of your first class (strength for a fighter) and 17 of your secondary class (intelligence for a mage). Keep in mind that, considering the level your char will reach (single class mages reach lvl 31 in TOB), your secondary class is actually the main one, in regards of planning and role in the team.
One last thing to consider is that most classes gain most of their defining abilities/characteristics on their first 10 levels. Add to that that the XP you sacrifice compared to the cap is very small (lvl 9 fighter takes 250k XP, the cap is 8m), you can see why duals are a powergamer's favorite (8m - 250k XP gives a lvl 30 mage instead of 31). You gain much by giving away little, in comparison to single class char.
Multies level up slower but get the full benefit of both classes, albeit lower than single class counterparts (the elf F/M would get 4m on each class ending up as a level Fighter 24/Mage 20).
I see now that this post got extremely long winded, sorry about that.
TL;DR: Multies get smoother but slower progression, duals require planning and have downtime but end as really strong if you pick complementing classes.