On the weapons tab, each manufactured weapon that has "hands" has two boxes next to it. Checking one or the other box means only 1 hand is using the weapon. Checking both means both hands are using it.
You'll find that creatures with natural attacks has a natural attack removed when equipping a manufactured weapon in either hand.
The changes when you equip/un-equip the weapon happen in the Armory Summary under the Weapons section. As well as in the weapons tab itself under "Other Weapons".
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