Arawn and Pwyll are such examples of split souls. Whereas Pwyll lived in the foreworld, growing up as a normal kid, into a normal guy, Arawn was left floating among the dunes. Eventually, he latched onto a poor soul who had failed the scales test and was flung into the foreworld, where he silently used people to get around until he possessed a horse skull and made himself a metal and cloth body. He soon founded the skull pantheon for lost souls and wound up becoming Lord of Death after Gwyn Ap Nudd, the former Lord of Death, somehow died. He and his fellow skulls collected the souls of those of the living splits and took them safely back to the afterworld.
On one such venture, Arawn and Pwyll met and instantly knew who each other was. There is not such as a knowing when you meet your soul mate, bur more of a remembering. After all, they were once you and your subconciousness remembers that.
Soon, Pwyll was pulled into collecting souls and delivering them to the afterworld.