This question isn't as much about NWN as D&D rules: in particular the inviability of most evenly-split dual/multi-class characters with equal amounts of each non-prestige class.

First consideration is that a good number of classes flat-out suck if you aren't going all the way on them. Anyone who's read the description of monks should know that they shouldn't be dual-classed (except if you're taking 1-2 levels in something for a few free feats/perks).

Casters are even worse on dual-classing due to how they scale cubically with level:
-Higher offensive DCs. A 10 fighter/10 cleric will have problems with enemy saving throws at level 20 that a level 20 cleric largely avoids.
-Access to better spells. D&D makes the mistake of making spell levels below 3 mostly non-combat utility, buffs, and roleplaying fluff. If you aren't going above the 2nd spell level, stick to being a stage-performing illusionist and hitting enemies over the head with a club. That's how bad the balancing and design is.
-The obvious: damage scaling. Every decent offensive spell scales with level, at least up to a limit.

The main rule of thumb when determining multi-class inviability is how many benefits are offloaded to the later levels. A 10/fighter/10 rogue might very well be viable as neither of those delay their main perks to later levels. Multiclassing with monk or caster might be somewhat useful if going to epic levels and gets cubically worse as the level limit decreases.

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So pick a designated hybrid class?...

Except that the designated hybrid classes (paladin, ranger, and bard) suck as well. Rangers in NWN1 are known to be comically weak and they only go up to spell level 4 with no nukes. NWN2 vastly improves dual-wielding making them somewhat useful.

Paladins are another comically weak class (compared to fighters or clerics for their respective specialties). Instead of fighter bonus feats, paladins get increased saving throws,disease immunity, weak healing, weak smiting, and turn undead. Paladin spell list only goes up to level 4 and has no nukes. Even worse, you're locked to 1 alignment. My advice: you aren't getting fighter bonus feats so you might as well be a cleric. The saving throw bonus looks even worse once you factor in the Champion of Torm prestige class having both saving throws and fighter bonus feats.

Bards are the only hybrid class resembling something useful. Team buffs, healing options, plenty of annoying debuffs and useful buffs - and ice storm at level 16. Despite these, you're better off with a fighter and mage and should only swap them out for a bard if you're strapped for party members.

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The main use of having multi-classes

Most of the good builds are main class/prestige class/bonus feature. Main class determines character archetype and role, prestige class enhances or partially redefines it, and the bonus feature either facilitates the prestige class or adds early bonus feats/perks.

Examples:
Cleric/Arcane Archer/Wizard with 1 level of wizard specifically to qualify for arcane archer -> heavy-armor self-healing mobile artillery with coaxial gatling bow (aka the Mako in Mass Effect)
Fighter/Weapon Master/Rogue with a few levels of rogue for sneak attacks, disarm traps, and open locks -> painful critical damage with sneak attack thrown in to be extra obnoxious
Wizard/Shadowdancer/Fighter using 2 fighter levels for free dodge/mobility -> nuker that dodges everything
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How this could be fixed

The paradigm needs to change. In particular, hybrid characters should be rewarded rather than punished. The existing design gives you the choice of (for example a fighter/cleric considering hybrid class): 100% competent fighter, 100% competent cleric, or 50% fighter/20% cleric competency. A proper and well-designed choice would be: 100% fighter, 100% cleric, or 70% of both (sacrifice a significant amount of power for multirole flexibility). The original Guild Wars MMO handled this well, with attribute level costs going up with higher levels. You could max out 2 attributes (level 12), leaving your build with serious deficiencies. More balanced (and reasonable) 3-attribute builds are 12/9/9 or 12/10/8.

Congratulations on taking your first step down the road of "I must fix D&D!" Thousands have trod down this road before you, and it is likely that thousands will follow you.

From here, have sprung, in one way or another, almost every single role-playing game that has ever existed - Albeit often rather removed these days. Still, one cannot have a discussion of role-playing games without at least sighting this road.

Anyway, good luck, and watch out for sharp rocks and suspicious ground. :)

i think DivisionbyZero makes some good observations.

i also tend to believe DnD is a foundation toolset on which you, the DM or player, builds realms. In truth, you can add or change any spell/class/ability combo you want. It just so happens that in the foundation realm TSR set up, that low level classes do get fairly basic abilities, and that combo classes say 10/10 or 20/20 will not access the highest abilities of each class.This is probably working as intended. Imagine a magic realm where level 40 spells are so scarce and hard to get that a character decides to forgo them and instead acquire abilities from another class.

however, even where that is the case, i tend to disagree with other conclusions of DivisonbyZero:

1) You may say that 10/10 or 20/20 or multiclass combos are not useful or optimal. This is true in some pvp settings. It's particularly true in max level 20 pvp. But is less so in max level 40 pvp. I can imagine viable 10/30 builds in level 40 team pvp. They may not be the very best for every situation, but they can be fun to play.

2) There is some suggestion that Ranger or Paladin is not a useful multiskilled class compared to cleric. This is not true. A Paladin is a dedicated spellcaster killer. A well built paladin is the fastest killer of spellcasters in the game (except perhaps another spellcaster, but even then a paladin will have higher chance of success). Basically, a Paladin is a magic-user killing machine. I have sen it time and time again in pvp. Paladins crush mages and clerics. Mainly because of Holy Avenger's on hit dispel properties but other Divine based abilities make the pally the most feared melee opponent in the game

As for the Ranger--- this is a deceptively subtle class to play. Think Aragorn. Strong, durable adaptable. That is the Ranger. Takes some thinking to take down your opponent, but if you do not kill the ranger fast, chances are he will figure you out and bring you down. I've seen this happen in pvp too.