@needfulnickel132 It's not an amp or a buff. Just a concept. Naruto and Sasuke have traits and abilities that exactly complete each other, making them, as a duo, godly in every aspect. Sasuke's intellect, observation and versatility + Naruto's wit, instinct and bodily strength make a perfect pair. Their innate teamwork and spiritual connection makes their completely different set of powers work as a whole.
That's why, in unison, they can defeat enemies that are stronger than both of them combined. The best solution for their opponent is to keep them separated, like Kaguya did.
The Biju Susanoo is when, additionally to their synced minds, "merge their bodies into one". In that state, their synergy works at best and it is impossible to differentiate them from a single entity. This was shown against Momoshiki who was already stronger than Naruto before eating his father, but became twice as strong as him after. Though his superiority to Naruto and Sasuke was clear in every aspect, they still managed to match him. And with the Biju Susanoo, they even outperformed him.
Yamitsuki is well aware of all that, and is not interested in becoming an enemy for the duo like Kaguya or Momoshiki.