Dualcast, also known as X-Magic, is a command ability in Final Fantasy VI. When Soul of Thamasa is equipped, Dualcast will replace the wearer's basic Magic command. Dualcast allows two spells to be cast in a single turn, doubling the turn-efficiency of spellcasters, but disables summoning espers. This can both boost damage output and healing capacity, making it an extremely valuable ability.
Dualcast can be used by a party member with Soul of Thamasa equipped, at which point it will replace command (meaning if Gogo has the relic equipped, Gogo will need to equip the Magic command to have access to Dualcast).
Dualcast is an especially powerful ability that makes spellcasters very turn-efficient. This can be used in a variety of defensive ways, such as casting Reraise on characters faster, or offensive ways, such as casting two Ultima or Meltdown spells at once. Which way the player uses it will likely depend on whether the player prioritizes curative spells over damaging ones. Using Quick with Dualcast allows four spells to be cast in one turn, plus a fifth spell if it is cast before Quickcast. Dualcast disables the ability to summon espers, but the tradeoff is overall worth it given the potency of spells that can be used.
When used in an offensive capacity, Ultima or Meltdown can potentially deal the highest damage output per turn, when facing formations comprising five or more enemies. If the group of enemies is four or less, the Master's Scroll and Genji Glove has higher damage potential (short of combining with the Quick spell), although Dualcast will still be extremely strong. Dualcast can also be used effectively with spells that target an enemy's elemental weakness, likely dealing equal damage to a powerful spell such as Ultima, but with much less MP cost.
In a defensive capacity, Dualcast can be used to cast Reraise on the party quickly, casting Reraise on all party members in two turns (or using Quick twice to cast it all on party members in one turn). Similarly, when a party member is KO'd, they can quickly be healed with Curaga when Reraise restores them; if they have lost Reraise; the player can cast Arise and Reraise in one turn to both fully heal them and give them Reraise again.
Dualcast can also greatly help with MP use due to the ability to cast Osmose. If a spellcaster is low on MP, they can use Osmose before their chosen ability in the same turn, or simply cast Osmose twice, to recover their MP. If the spellcaster uses Celestriad in addition to Soul of Thamasa, MP is no object (short of using a weapon that drains MP), meaning they become a powerhouse with few drawbacks. The problem with this setup is that it prevents the user from equipping defensive relics such as Ribbon; as such, in a defensive capacity, it may be best to instead use Dualcast and having a defensive relic.