psychophipps
Explorer
Presumably, it just means you combine the damage die of each weapon for every iterative attack.
Compare to a maul user. Someone with a maul would do 2d6+Str bludgeoning damage with every hit, while someone dual attacking with say two maces would also do 2d6+Str bludgeoning damage with every hit. I'm not sure how it would work if you mixed damage types, nor how to prevent weapon combinations from doing more than 2d6 in total weapon die damage.
Why use twin maces in your example? Dual Wielder specifies they can be any one-handed weapon. Longsword, Battle Axe, and Warhammer all are 1d8 damage and Versatile in case you get one disarmed.