It seems that a cleric dualed to a mage is a favorite power character in many of the other games here, yet I've searched in many combinations of the terms on NWN forum and found nothing.

Is that multiclass doable/advisable/powerful in NWN?

Problem is in 3rd edition the power of spells themselves is actually dependent on the spellcasting stat rather than just the number of spells learnable or per day. Clerics and wizards rely on wisdom and intelligence respectively and as such you'd need to split your advancement between the two. Also, levelling is different from the third edition games so an equal split of caster levels means the character would be way behind a pure caster in terms of spell levels known, not to mention the power of those spells.

I think the only way it would really work is to have, say 4 or 5 cleric levels and the rest mage. Clerics get a lot of nice buff spells (for which caster level doesnt really matter too much) at low levels as well as weapon/armour proficiencies and domain abilities, which can toughen up a mage quite nicely. Also, taking the buffs as a cleric would free up spell slots for arcane spells. Sorcerer might be an even better choice, as with Charisma as a casting stat you'd get some synergy with the cleric's domain abilities, increasing their duration, and could even turn your low level turn undead into something slightly useful. Well, it could at least provide a convenient way to blast through roadkill undead.

If you're taking the character epic, 10 levels in an associated prestige class couldnt hurt either. At 40th level, a cleric 5/sorcerer 25/Red Dragon Disciple 10 would be a fairly good arcane caster who can hold his own in combat as well. Take the Automatic Still Spell feats so you can use armour while casting, load up with your cleric buffs, arcane reciprocal damage spells and summons and you're away.

Of course, its not really the same as the 2nd edition cleric mage, but thats the nearest thing I can come up with :)

The main problem is that you lose spellcasting power. You cannot level both classes at the same time.

Unless you download and install the PRC Pack (http://nwnprc.netgamers.co.uk/) and use the Mystic Theurge PrC.