Then you haven't understood the situation.
Cabins A,B,C,D get allocated to the 11 nighter.
Cabins E,F,G,H get allocated to the split itinerary.
That's what the booking system gets told and the software isn't designed to allow willy nilly selections between those cabin groups, it would be too chaotic to program.
It might be possible to do it though a Princess Vacation Planner or a big and very good TA, who have the knowledge/ability to manually over-ride the booking system.