Also known as soul mates, split souls are literally souls that have split into two or more soon after they formed in the Duat. The cause is unknown, but it seems to occur if the soul is in turmoil; something harassing them, upset in the foreworld, etc.
Once the soul has split, one half leaves for the foreworld to live out the life the full soul would have had. This half has very little, if any, connection to the Duat. Usually, a soul will live its entire life in the Duat and only have a fraction of itself in the real world. This half will also have no reflection, due to having no connection.
The half that stays in the desert has no physical form, but it is alive and aware of its surroundings. They will usually seek out a living organism or an object to possess.
These two souls can still have a connection to one another, and may leech powers and abilities and even memories off of each other.
Split souls who die in the foreworld will not wake in the Duat like normal ones do. They wake in the living world, often in the same town, same room, in which thethey died. It is Arawn's job to collect these souls, as it is a more common phenomenon than you'd think.