The discussion is about shore excursions, not speciality dining.
Regarding pre-booking shore excursions for more than one cabin, I had to book one cabin, checkout and pay and then repeat the process straight away for the other cabin. The discount was applied according to loyalty level. On our last cruise with our grown up children, the eldest was a lower tier than the other two and it appeared automatically. I would imagine that the same thing would happen if I were to book any excursions onboard.