Hmmm as a light weapon it should be able to be dual weilded. However NWN has made it so it can't for some reason. Not sure if its intended or just a bug that hasn't been fixed.
Try reporting it to Bioware.
A way around this would be to make yourself a flail in the toolset but give it the item tag of another light weapon. That way it "could" be armed in the offhand.
You would have to take your weapon prof's though in the weapon that you set the item tag to.
I have never tried this but I can't see why it wouldn't work.
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