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No, you cannot do both. If you have booked for Club Dining you cannot decide to go to Freedom Dining. However, you can choose to eat in one of the Select Dining choices - depending on the ship. I assume you are going on Ventura or Oceana as they are the only ones that have Freedom Dining. There is usually a buffet you can go to without a reservation.
Hope this helps.
If you decide you would rather have Freedom dining you can either give P&O a call and ask if you can change or alternatively the day you arrive on board go and see the Restuarant Manager and ask if you can change.
Usually more people want Club Dining than Freedom Dining so you should have no problem changing.