Another factor to keep in mind is that if the twin they spare later becomes a ghost themselves, their hun and po souls are quite likely to then engage in mutual symbiosis/cannibalism with their Yuu-Zin counterparts, the quartet devouring each other until their collective mass melds into a single being. The family's domain in the Underworld is controlled by a trio of such beings, which drive their living descendants to continue the tradition so that future twins can have the same chance at true spiritual unity.