Synergized is a gameplay feature in "Episode INTERmission" in Final Fantasy VII Remake Intergrade where Yuffie and Sonon combine their strengths at the cost of ATB filling slower. This can be mitigated with Haste, easily applied to both party members with Time Materia linked to Magnify Materia, and by using Ninja Cannonball.
To synergize, both Yuffie and Sonon need to be in the party; Sonon joins during "The Handoff" in "Wutai's Finest". The party can then synergize by tapping when both party members are alive. The same button releases the synergy. When the player synergizes, Sonon will appear next to Yuffie no matter where he was on the battlefield previously. If the pair gets separated during synergy, the player can release the synergy and then immediately reapply it, and Sonon will again appear next to Yuffie.
When synergized, Yuffie and Sonon focus on the same opponent and Sonon will match Yuffie's damage type (physical weapon strikes or magical ninjutsu), useful for racking up damage on a specific foe and for building stagger. When synergized, Yuffie's and Sonon's health bars and name have a blue glowing edge in the battle UI.
When synergized, the pair can use Synergized Art of War, Synergized Windstorm, Ninja Cannonball (with Ninja Cannonball Materia) and (with Sonon's Marshalist's Staff sufficiently upgraded), synergized limit break. To use a synergized limit break, Sonon needs to equip Marshalist's Staff with Synergized Limit Breaks ability, Yuffie and Sonon must be synergized, and both need their limit breaks ready. The final condition is most easily achieved by equipping Götterdämmerungs. If Sonon has Synergy Materia with a Magic Materia, he will also cast a free spell in response to the pair using Synergized Art of War or Synergized Windstorm.
Ninja Cannonball does not use ATB, instead being the duo's joint "Attack" command, which builds Sonon's ATB if it connects. It is good to make use of to build ATB for the other synergized abilities.
Synergized Art of War and Synergized Windstorm expend both party member's ATB, but the cost can be reduced by equipping a Kindred Cord on either Yuffie or Sonon. The optimal time to use a synergized move is when the player needs to deal a lot of physical damage to an enemy or to stagger one. Synergized Art of War deals powerful damage against a single target, as well as any enemies adjacent to the one being hit, while also empowering the next ability. Synergized Windstorm only has one form (whereas the normal Windstorm has a melee and ranged forms), dealing area of effect Wind damage around the pair while building the stagger damage multiplier against an enemy that is already staggered. Synergizing thus shines especially during boss fights.
To build ATB specifically for the synergized abilities, the player can release Synergized as soon as the pair finish their move, build the ATB back up faster while not being synergized, and then synergize again to pull off an ability. However, being synergized has many benefits outside of the specific compatible abilities. The pair is not as easily interrupted and recover from being knocked down faster, Sonon catching Yuffie if she is blown back by an enemy's attack; the player can hit as soon as Yuffie is knocked back for Sonon to catch her and negate all immediately followed damage. The pair can double up their magical ninjutsu skills, which is especially good for staggering some enemies with an elemental weakness after Yuffie has used Elemental Ninjutsu. Simply attacking a target when synergized pressures/staggers them easier. Precision-guarding while synergized causes Sonon to counterattack Twirling Lunge with no ATB cost.
During the final boss, Yuffie and Sonon need to be synergized to avoid Yuffie being instantly defeated by one of Nero's attacks.