As Dzug states, the only requirement is to have one executor. However, it's often sensible to nominate a 'reserve' executor, in case the prime executor is unable to take on the task or declines to do so. (Nobody can be forced to act as executor).
For example, the daughter might die in the same car crash as her father. Alternatively, her father's death might occur at a time when she is already under great stress (e.g. because her own child is terminally ill) and she may not want to take on the task of acting as executor for her father's estate.
Chris