If you get to be one of the last ones to check in then there is a possibility you could get seperated. It used to be that you could just turn up and Thomas Cook would try to seat you together but it seems that they have introduced a way of getting more money off you with " your seat, your choice " price is £12 each.
Princess do not use the service where you can pre book your seats on the link you posted, they have said that it should not let cruise people do this and if it does it makes no difference and it will not book your seats together. They are looking at doing this in the future but not yet.
Princess advise to get to check in early as its first come first serve so if your at front of que no problem and if your at back you high chance you will be seperated so we are planning on getting there earlier than normal.