The off-hand dragon scimitar can be bought either from Daga's Scimitar Smithy on Ape Atoll, or from Dealga's Scimitar Emporium in Varrock. It should be noted that if you wear this weapon you cannot wield a shield, since the off-hand dragon scimitar is wielded in the shield spot. If you are dual wielding and do not have a shield equipped, there are some abilities that are not accessible. This is made up for by the abilities that require you to be dual-wielding melee weapons in order to perform them. The off-hand dragon scimitar, like the regular dragon scimitar, requires completion of the Monkey Madness quest in order to equip it. It can be bought from Dealga or Daga for 100,000 coins.
The off-hand dragon scimitar hits half the amount of damage as the regular dragon scimitar.