As the name implies, Soul Multiplicity combines two or more souls using Idle Transfiguration. It also creates a reaction due to the soul's naturally rejecting the fusion with one another. This allows the user to derive more techniques that take advantage of the reaction.
In Shibuya, Mahito used Soul Multiplicity to lead into his Body Repel technique, which uses the rejection of fusion to create an overwhelming output of transfiguration. Unleashing Body Repel mercilessly cut down countless non-sorcerers as a way to keep his adversary at bay.
Soul Multiplicity takes a little bit of time to charge. During his rematch with Yuji, Mahito used a transfigured human to occupy Yuji while the curse got behind him and combined two distinct transfigured souls together. This once again leads to Body Repel, but creating a much more focused attack that combines the multiple souls into one transfigured monster attack.
In order to counter Aoi Todo's Boogie Woogie technique during their fight, Mahito combined numerous miniature transfigured human souls together at once with Soul Multiplicity. Hiding several in his stomach, Mahito vomited the transfigured humans before crushing them all together and unleashing Body Repel to attack Todo from all directions.
When that strategy proved ineffective, Mahito used Soul Multiplicity to merge souls with weak rejection together to form one body. This generates a Polymorphic Soul Isomer, an unique type of transfigured human. Mahito attempted to use the Polymorphic Soul Isomer to distract Todo with its explosive attack power. While it was fighting Todo, Mahito used Soul Multiplicity again to pressure Yuji one on one.