This iconic and flavorful mode originates from the Season of Magic and encourages you to go all in with special cards. So you want to pack 12 special cards that you can include into a deck and add some units that play specials on their own. That way, you avoid missing out on the double casting effect most of the time. However, you only get to duplicate the first special you play each turn, all following specials will only be cast once.
As you might assume, removal is quite prevalent, so engines will have a hard time. Removal, both of the tall and the wide variety, tends to be abundant, so it often comes down to fighting for last say.
And there is one very popular finisher that three factions have access to: Harald Gord.
This card is usable only by Scoia’tael and Syndicate, but Nilfgaard, having access the Double Cross leader ability and Bribery, sometimes manages to utilise Gord as well. No wonder that these three factions seem to be the most popular ones during this seasonal mode. Not only because of Harald Gord, but also because they have fleshed out archetypes with Nature/Spell cards, Crime cards and Tactic cards. An Arachas Swarm list from the Monsters faction can also be considered a nice archetype for this game mode, since it tends be be quite special card heavy while not providing great removal targets. Northern Realms doesn’t seem to offer much at first glance, but an unconventional deck with mages, spells and Cintrian Royal Guards has proven surprisingly effective. Skellige is pretty rare, but there is a Lippy deck that focuses on duplicating Shupe as much as possible.
So while the Dual Casting mode can be very punishing and control heavy, it is surprisingly versatile with all factions having something viable to offer.