There can be one personality that can be split into two personalities. People with multiple personality disorders are not born with the disorder. The same with people whose corpus callosum is severed. It is fluid. The amount of damage to the brain will determine how distinct the personalities will be. If you could transplant brain it possible that one personality could create two fully distinct personalities. The process could be in principle also reverted back again leading to one personality.

Additionally, the process of separating parts of a brain is also continuous in time. People could be experiencing qualia when then brain is being damaged and the process could take long enough for them to register the states in between.

With regard to reports, here:

> He also reported to have grabbed his wife with his left hand and shaken her violently, at which point his right hand came to her aid and grabbed the aggressive left hand.

And here: https://youtu.be/N1Mac4FeKXg?t=124

With regard to definition of personalities - definitions created for an average case are often not sufficient to describe the reality. Quantum physics is a perfect example how reality can break old definitions.