Ghosts have the ability Malevolence that allows them to possess a creature.
This ability is similar to a magic jar spell (caster level 10th or the ghost’s Hit Dice, whichever is higher), except that it does not require a receptacle ... If the save fails, the ghost vanishes into the target’s body.
However, it's unclear what happens when a ghost is possessing a being, but another ghost wants in on the fun. Magic jar specifies what happens to the soul currently occupying the body - it goes into the jar. But Malevolence does not use a receptacle, nor does it specify what happens to the original soul.
So what happens when a ghost successfully uses Malevolence on a creature already possessed by another ghost?