Hi Vijay Kumar,
During the Pre-separation action you want the position occupied by person A to become vacant so what you do here is while running the action assign the default position to the Person A so that automatically the position occupied by Person A will become vacant from the date of Pre-separation.
So after that assigning the vacant position to Person B or X does not matter as this could be done easily just assign this position while hiring a person during the hiring action even before the other Person gets separated.
But make sure you hire a replacement person after you run the Pre-separation action for the person who is about to separate.
Regards,
Mithun K