Well, more of an ability.

!Spell Fusion: By spending 1/2 of the mp cost of two spells, combine the spells into one to create a unique effect.

So say you want to cast Firaga and Thundaga. You combine the two into something entirely new, in this case something like Firestorm.

Works for other spells too. Meteor and Ultima? Meteoric Annihilation.

Not quite as powerful as X-Magic, but provides for more interesting combinations.