hercufles: I was wondering since its been a while that i played advanced dungeons and dragons and never used dual class. But i noticed (maybe its tutu) that coran is a lvl 6 fighter rogue doesnt that mean is a lvl 12 character? If you add those 2 together?

Jarmo: No, that'd be if it were with 3.x rules, in BG's 2.x it's different.
Basically you get a 6th level dual- or multiclass for the same XP you'd need to reach 7th level in a single class.
The XP needed to reach the next class is independent of how many levels you already have in some other class.

End result, dual- and multiclass characters are generally more powerful than singleclass characters in 2.x rules,
and less powerful in 3.x (NWN, Elemental Evil) rules.

hercufles: Maybe more powerful but more restricted, for example if you gonna play cleric/warrior you still cant use other weapons then mace.

Jarmo: Oh dang! I'm so unused to 2.x I forgot that. Guess a few levels of fighter and a longbow mastery (being what I'm using right now in a new game) isn't such a hot groundwork for a mage. D:

mystral: The weapon and armor restrictions only apply in 2nd ed to cleric and druid multiclasses.

mystral: The weapon and armor restrictions only apply in 2nd ed to cleric and druid multiclasses, because those restrictions are for faith-related reasons, so a cleric can't use a blade even if he knows how to use it, and druids can't use plate.

OTOH, a wizard who gained a weapon or armor proficiency through some levels of fighter or anything else will be able to use those. He can't cast spells while in armor however.

Dual class characters are more powerful than a single class one, as a rule.
For multi classes, you will be several levels lower than a pure class, so it's a mixed bag mostly.

Jarmo: IMO, 2.x dual class system sucks salted chocolade balls.
It's effective, but "now you can lockpick, now you forgot how to do that, now you remember again".

gnarbrag: That's the BG interpretation of dualclassing. In PnP you can still access your old skills but you get an xp penalty for encounters during which you use you old class.

Jarmo: A few more thousand XP and my mage can also give master-level fire support with a composite bow. Should be fun.

I almost always mix and match classes at least a bit. Much simpler to do in 3.x rules.
Actually if I had thought about it longer, I'd probably have taken an elven multiclass fighter/mage, but here we go...