The Double-Bladed Scimitar can be found in Eberron: Rising from the Last War, page 21.
An alchemical process has bonded silver to this magic weapon. When you score a Critical Hit with it against a creature that is shape-shifted, the weapon deals one additional die of damage.
If you attack with a double-bladed scimitar as part of the Attack action on your turn, you can use a bonus action immediately after to make a melee attack with it. This attack deals 1d4 slashing damage on a hit, instead of 2d4.
This weapon requires two hands to use. This property is relevant only when you attack with the weapon, not when you simply hold it.