The Whetstone item, which is rather common, adds +1 damage to a weapon, but it seems that it doesn't really respect the fighter's level 5 ability (when the dual-wielding perk is chosen) to give the extra damage to both wielded weapons. Could this be changed?
Just to add to this, keen edge also does not duplicate to both weapons, nor does sonic weapon rune that I can't think of the name of right this second.
"The effects of Magic Weapon, Flame Weapon, Darkfire, Poison and Alchemist's Fire duplicated to off-hand weapon when cast on your main weapon" says in the perk, for a particular reason. We do not really want to add it to work on everything possible, just these particular ones folks may self-apply and are more common. Whetstone I could see, totally, but at the same time it's also rather easy to just apply on both and RP it a wee bit.