It all depends on how realistic or customizable you want the clothing system to be.
If we’re talking realism and upmost customization, make it 2 glove slots. You can have gloves in 1 slot (still applies to both hands) and then the brass knuckles over the gloves in the 2nd slot, because nothing is stopping you from using brass knuckles and rings over gloves, brass knuckles are wide enough.
If we’re talking less customization, but still realistic, you can have gloves in the gloves slot and knuckles as a 1 handed weapon.
BUT, i think that things like knuckles shouldn’t take away from using other weapons. You can still grab a 2-handed weapon like a fire axe, with knuckles. Yeah, it’s more uncomfortable, but it works 100%.
I personally think that knuckles should be used as a cosmetic in the gloves slot that grants increased punch damage. Early game players can have higher damage, people later into the item progression can use gloves instead and have a weapon of their own. Brass knuckles don’t seem like an item that would be viable once you’ve geared up, so it ends up being a cosmetic decision between gloves and knuckles, UNLESS gloves have attributes similar to what i mentioned in this post. If gloves have attributes of their own, then it turns into a “should i look cool with unpractical knuckles ?” or “should i ditch the knuckles and use gloves for their fall damage reduction ?” decision.