(Sorry I didn't reply sooner! Whole computer froze and I sorta…. typed a lot… lol)
That's a cool concept! Really creative, I didn't even think of a fuse system where souls are split and shared. Though I don't want to steal your idea and it would contradict a ton of stuff in my world. If souls are split it my world it either makes you incomplete or dead for the most part. Before I go into that I'm going to discuss Ben and Ruth. Ben, though he has a strong will and hard-working, has certain fears or things that cause quick reactions. Ruth used this to their advantage when Ben was split off from a group he was with, using fear mimicry to make him react. Though he reacted in anger rather than fear it still was used to trick Ben into allowing their magic to latch on to his soul, creating a bind despite him not being the one the represented the fear of. This makes it more unstable which can cause several side effects, the main one being Ben having to keep his emotions in check even if he is level-headed for the most part. It also keeps Ruth from having to be loyal to Ben, taking control of his soul and body if they ever felt it's needed (Not very often) and even allowing itself to make alliances with those like Aaron.
Now the soul part so I can explain how they work. Pretty much everyone has a soul that is represented by a flame. The color, whether the flames flicker in jagged motions or softly, and how strong it glows are all determined by the owners personality and health. If a soul is split it often is very small and weak or instead is seen as an item that the owner carries or wears at all times, making them vulnerable if they don't know what they are doing. If one were to know how souls work they could essentially revive themselves with the help of another, the two dead or almost dead souls making a whole in which one gains control of their body with the aftereffect being the presence of another that may or may not be dormant. This is Aaron's situation with Corru, and yes I did just say Aaron was dead at some point. Incomplete souls are often the aftermath of failed revivals of a person or entity, like a character I have named Revi who has a necklace that represents their soul rather than a flame and survives as a ghost. The drawback to this is that the person may not be fully there, such as Revi missing memories of who he is and though still wise may not know why he has knowledge of some things.