The Twin Holsters is a Craftable Item in Assassin's Creed III
Will let you dual wield two firearms, greatly increasing long range damage potential.
You need to equip a sewing thread to make this, you can craft under "Materials", you need wool for it (From farmer)
This recipe is unlocked after doing the Homestead Mission Silk Errand
Twin Holsters Recipe - Twin Holsters require the following materials to craft:
From Hunting or from Huntress