the feat should add, that you can make 2nd offhand attack if you have Extra attack feature.
In A5e, the rules for the Two-Weapon Fighting style do allow you to do this when you use your bonus action.
Two-Weapon Fighting
When you take the Attack action and attack with a weapon that does not have the heavy property that you are wielding in one hand, you can use your bonus action to attack with a different dual-wielding melee weapon that you are holding in your off-hand. You do not add your ability modifier to the damage roll of the bonus attack, unless that modifier is negative. If a weapon has the thrown property, you can make a ranged weapon attack with it instead.
If you have the Extra Attack feature, you can use your bonus action to make two attacks with the weapon in your off-hand.
The A5e version of the Dual Wielder feat:
Dual-Wielding Expert
You’re a master at wielding two weapons at once, gaining the following benefits:
• You gain a +1 bonus to Armor Class while you are wielding a separate melee weapon in each hand.
• You can use two-weapon fighting with any two one-handed melee weapons so long as neither has the heavy property.
• When you would normally draw or sheathe a one-handed weapon, you can instead draw or sheathe two one-handed weapons.
With the offhand attack now a part of the Attack action, how does the Extra Attack feature in One D&D work with the Two-Weapon Fighting style? Do you go from making two attacks (from levels 1-4) to making four attacks when you get that feature?
curious