Aerith clones Yuffie, who launches a sprightly assault.
Description
Mystic Multiplication is a synergy ability in Final Fantasy VII Rebirth, used by Yuffie Kisaragi with Aerith Gainsborough as a synergy partner. It costs three synergy charges each, performs magic damage to a target, and grants them temporarily unlimited MP for a duration, as well as instantly filling Yuffie's ATB gauge; it is the only synergy ability to grant ATB refill and this is not mentioned in the folio or ability description.
In the ability's animation, Aerith clones Yuffie whom the latter sends at the enemy, while Aerith fires a ranged blast at them. Meanwhile, the original Yuffie and Aerith charge up a larger magic orb which they fire at the enemy.
Obtained[]
Mystic Multiplication is unlocked on Yuffie's folio, and costs 20 SP.
Use[]
Mystic Multiplication costs three synergy charges each on first use, and four on subsequent uses, and will grant Yuffie and Aerith unlimited MP for a duration. This allows them to freely use spells, limited only by ATB charges and not by their MP, and by the spells they have in their equipped 
Magic Materia. Aerith has the strongest 
Magic Attack and MP pool in the game, while Yuffie's balanced attributes and Doppelgänger make her a very strong potential spellcaster.
Both Yuffie and Aerith have another synergy ability, each with Cait Sith, that can grant unlimited MP. Yuffie with Moogle Pinwheel, and Aerith with Kitty Cannonade.
Yuffie's Doppelgänger, when deployed, will cause her spells to be cast twice, albeit the second time at reduced damage. This, and her ability to build ATB quickly, makes her a strong potential spellcaster, and giving her unlimited MP can unlock this potential further. Combined with Aerith, this can be especially lethal: Aerith's Arcane Ward causes all casters within it to cast spells twice, and coupled with Yuffie under Doppelgänger, she can cast spells four times. Any 
Synergy Materia users within her ward also cats spells twice, and she can quickly pull others to her ward with Bodyguard. As such, using Mystic Multiplication can be exceptionally strong. It is best used when Yuffie has no ATB as it will instantly fill it for her.
In Hard Mode, gaining unlimited MP can be more crucial as means to restore MP are more limited. The player may also have the rare and expensive 
Reraise spell available, perfect to be cast during any MP-free period.