This would be a very large undertaking which I can only advice against. The game does not use equipment slots - at least not in the traditional sense.

Weapons are only equippable in "one slot", either (mainhand), (offhand) or (mainhand+offhand = two-handed weapons). This is set in the item. So if you have say the weapon named "Axe_Hunter" which is a mainhand weapon, you can only ever equip that weapon in the mainhand. Because of this, what you would need to do to have off-hand weapons is create duplicates of all mainhand weapons in the XML. With such a solution you could tweak your off-hand, but an item would still be defined as either mainhand or one of the others above. So you'd need to have two weapons in the game - one called Hunter's Axe, and another called Hunter's Axe (offhand). Switching one to the other would not be possible.

In addition, animations are sort of hard-coded to treat any off-hand-only equipment as a shield.

Something that is a lot easier to add is weapon pairs like the Trogs have with their axes. You can't switch out weapons in the pair, but it is a very simple thing to make, say, a rapier in one hand and a small crossbow in the other hand (for a shadowy assassin!). They would just be treated as a single item instead of two separate items.

Just a quick example from Stormworld with some altered details

 <GameItemType InternalName="Reliquary_Demonslayers">
    <DisplayName>Demonslayers</DisplayName>
    <Description>Reliquary Item #72. Two small scythes. The handles are adorned with what appears to be human spines and the handleknobs have tiny skulls attached to them, possibly shrunk human skulls but small variations in the structure imply that they are demonic in origin.  The scythes once belonged to a heroic Slayer from the Gnome kingdom. The Slayer devoted her life to seeking out evil in all its forms in order to restore her honor which had been tarnished in the past. The Slayer was so successful in her endeavours that her name, Gorita, became incorporated into children's tales. Some even said that the weapons had become feared by the demons of the other worlds, although that is likely not true. What is true, however, is that the scythes have a special ability that makes them useful when fighting demons. Even being near the weapons will cause demons discomfort, and if they are struck by the scythes a charged energybolt will strike them from the blades causing significant damage. Hosten's note: "I must admit that I have a conflict of emotions storing these in the Reliquary. I wonder if the world would not be better off if these were out in the world, doing what they do best."</Description>
    <Type>Weapon</Type>
    <Type>Defense</Type>
    <WeaponType>Blunt</WeaponType>
    <CanBeEquipped>1</CanBeEquipped>
    <ShopValue>700</ShopValue>
    <GameModifier>
      <ModType>Unit</ModType>
      <Attribute>AdjustUnitStat</Attribute>
      <StrVal>UnitStat_BackswingChance</StrVal>
      <Value>100</Value>
      <Provides>Backswing- If the attack misses the unit gets a second attack against the same target</Provides>
    </GameModifier>
    <GameModifier>
      <ModType>Unit</ModType>
      <Attribute>AdjustUnitStat</Attribute>
      <StrVal>UnitStat_Attack_Cutting</StrVal>
      <Value>18</Value>
    </GameModifier>
    <GameModifier>
      <ModType>Unit</ModType>
      <Attribute>AdjustUnitStat</Attribute>
      <StrVal>UnitStat_ChanceToCrit</StrVal>
      <Value>3</Value>
    </GameModifier>
    <GameModifier>
      <ModType>Unit</ModType>
      <Attribute>AdjustUnitStat</Attribute>
      <StrVal>UnitStat_CurrentWeight</StrVal>
      <Value>9</Value>
    </GameModifier>
    <IsAvailableForSovereignCustomization>0</IsAvailableForSovereignCustomization>
    <Likelihood>300</Likelihood>
    <RarityDisplay>Rare</RarityDisplay>
    <HeroOnly>1</HeroOnly>
    <IsAvailableForUnitDesign>0</IsAvailableForUnitDesign>
    <Prereq>
      <Type>UnitStat_Level</Type>
      <Attribute>8</Attribute>
    </Prereq>
    <AIData AIPersonality="AI_General">
      <AIPrefType>AIPrefType_CUTTING</AIPrefType>
      <AIPriority>500</AIPriority>
    </AIData>
    <ArtDef>Reliquary_Demonslayers</ArtDef>
    <GameItemTypeArtDef InternalName="Reliquary_Demonslayers">
      <GameItemTypeModelPack InternalName="Axe_Default">
        <IconFile>Reliquary_kultaxe_icon.png</IconFile>
        <TintR>0</TintR>
        <TintG>0</TintG>
        <TintB>0</TintB>
        <AttackSFX>Hit_Hammer1</AttackSFX>
        <AttackSFX>Hit_Hammer2</AttackSFX>
        <AttackSFX>Hit_Hammer3</AttackSFX>
        <EquipSFX>Equip_WoodenItem_01</EquipSFX>
        <EquipSFX>Equip_WoodenItem_02</EquipSFX>
        <EquipSFX>Equip_WoodenItem_03</EquipSFX>
        <GameItemTypeModel>
                    <ModelFile>gfx/hkb/Weapons/W_Axe_01.hkb</ModelFile>
                    <Texture_All>gfx/hkb/Weapons/FE_Weapons_01.png</Texture_All>
                    <Scale>1.2</Scale>
          <Attachment>hand_right_Lcf</Attachment>
        </GameItemTypeModel>
        <GameItemTypeModel>
                    <ModelFile>gfx/hkb/Weapons/W_Axe_01.hkb</ModelFile>
                    <Texture_All>gfx/hkb/Weapons/FE_Weapons_01.png</Texture_All>
                    <Scale>1.2</Scale>
          <Attachment>hand_left_Lcf</Attachment>
        </GameItemTypeModel>
      </GameItemTypeModelPack>
    </GameItemTypeArtDef>
  </GameItemType>

The higher the likelihood the more likely it will show up. His rarity should be somewhere between uncommon and common Most of the items with that rarity. Also, when a quest or goodie hut says to give random item it specifies uncommon, common, and such.