Publish 21 took away the ability of players to craft double slayers. Monsters have NEVER dropped double slayers, except in cases where they looted one from a player first. Almost all double slayers, except juka bows, are sword weapons from the old paladin quest that always gave a sword with undead slaying and some random properties.
Some of the ML special recipe weapons can be doubled.
A -USEFUL- double slayer, with a super slayer property that overlaps the other slayer property (reptile + dragon for example) to add 4x damage can sell for quite a bit- the old juka bows that became double demon or demon+elemental often fetch 15 million. Less useful double combinations aren't highly sought and typically bring no premium at all despite being technically rare.