Huh... it... shouldn't have. As in, I'm 90% certain I tested that way back when, and 99% sure I tested that more recently when I tested the deprecated functions using NifSE. I wonder why that bug would occur.
Oddly the problem never occurred with the deprecated functions on NifSE. I just assumed you'd fixed it on purpose

( that totally stinks that i can't use weapons to duel wield unless they are mod-added with my toon growing and leveling up constantly, i don't think having one sword sounds to practical as it might be higher overpowered or highly underpowered :'(
Get a replacer for the standard weapons (classic sword replacer is lovely) - i don't remember the link, but it's in the FAQ from the UV page. That way you'll still have the normal weapons and you'll be able to dual wield them - they'll just look a little better.