"Dual Wield Expert" is the name of the passive. Here is what it does:
"Increases Weapon and Spell Damage by 6% of off-hand weapon's damage."
So with a gold mace the damage of the weapon is 1335 (not weapon damage, damage of the weapon!)
With nirnhoned it would be 1535
With nirnhoned and the Heartland Conqueror buff it would be 1735
With that passive at level 2 I'd get 6% added weapon/spell damage of that 1735 (damage of the weapon) value which would be 104 extra weapon/spell damage from this dual wield passive. This passive has NOTHING to do with the main hand weapon in any way. It gives more weapon/spell damage based on the value of the "offhand" weapon's damage which is maximized by using nirnhoned (on offhand weapon) and having the Heartland Conqueror set (5-pieces) equipped.
Are you just trolling because you don't want people to be as strong as you or are you really not understanding what the passive does?
A legendary mace gives 1,335 weapon/spell damage. This is the same regardless of trait. So by making the main hand weapon nirnhoned you are adding 200 weapon/spell damage. By having the Heartland Conqueror set also you add another 200 weapon/spell damage. So that is 400 extra weapon/spell damage based on how people here said this is calculated. With major brutality/sorcery it would be 480 weapon/spell damage.
If you go sharpened instead you still have the 1,335 weapon/spell damage. Instead of adding 480 weapon/spell damage (buffed) from nirn and Heartland buff you will add 3,276 offensive penetration. You really think that 480 weapon/spell damage would give me more damage against a heavily armored player than 3,276 pen? I have time now, I'll go throw it in a build editor and report results.