The only thing we have so far is that Light “tried” to pull away Uther’s soul, but Frostmourne ripped away a part of it. However, that still leaves open questions:
what was going on with an image of Uther next to his tomb?
did those parts become one again?
Overall it’s not the first time such soul machinations happen. For example, there was a quest line in Cata involving Thrall being splitted. And in Helheim the player character had to collect one’s soul parts.