Technically, though NWN treats them as a weapon since NWN never included the ability to hold anything in your hand except a "weapon". So all the held items/accessories are "weapons".
Like Books.
Mugs.
...Babies.

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To answer your question though, yes, any one handed weapon (exceptions above) can be dual wielded.
Anything that is size Small wielded in your offhand gives the standard penalty (-2/-2 with Dual Wield Feat), while Medium (Rapiers, Longswords, Katana, etc) weapons in your offhand give an additional penalty, for a total of -4/-4 with Dual Wield feat.