Duel Wield Sword

this is not possible by minecraft design.

You have to make a main-hand version and a off-hand version of it. For the ofhand, make a procedure where right clicked on entity, deal [insert damage] to entity.

yeah but there won't be a swing animation and it won't work with mobs that already have right-click functionality like villagers...

I know, but that is the only thing I can think of. Plus, it isn't very likely (and quite cruel) for players to kill villagers. And I think there is a procedure block that moves an entity's arms?

it isn't very likely (and quite cruel) for players to kill villagers

yeah, who would do such a thing cough cough 

there is a move arm procedure block though, I completely forgot about that XD

but stikk the sword won't have hitting particles, sweeping edge, enchantments, etc.

Bad boi, why hit villagers?

There isn't custom enchants yet anyways, and I think you can spawn particles? Just say sweeping edge doesn't work. Nobody will question it. It is just part of the mod's design! (Totally not because I can't think of a solution.*cough cough)

Or you can check the player's velocity, and if it is 0 then check for entities on either side of the mob you hit, and remove some more damage. Use chance templates as well. (that is the requirements for a sweeping attack, right?)

Use the procedure "swing off-hand" for the swing and, in case of hitting a mob with inventory, use the procedure "close GUI currently open". I think that should work.

villagers have trades worse than Muffet from undertale

I mean it won't work with vanilla ones. btw custom enchantments are already added in 2020.4

Oh... maybe somehow summon an invisible mob that have what you have in the offhand that attacks for you and then suddenly despawns?

On player right click on mob:
check if player holds weapon offhand;

swing off-hand item, deal damage to mob
 

your ideas are way too overthought

still figuring out a way to handle the cooldown tho

I was making a Mod That Requires Offhand-Attacks but I can't use Source Entities for When a Player Right-Clicks with the Weapon so that won't work...