thats not true, only one +50FP package is active, but it is true that only one FP-package (+50FP) shows up on the character sheet.
as dark consular i ve got a mana pool of 270 (incl. +50FP from dark mastery consular) ,
changing into a prestige class like sith killer (assasin) after level 15 i ve got 270 FP + 3 DEX. (+3 DEX shows up on the char sheet).
but changing as dark mastery consular into a prestige class sith-lord, i ve got as always just 270 FP, nothing else.
(+ 50 FP shows up on the char sheet )
checkt it out , as dark mastery consular + sith lord, you ve got no extra bonus from prestige classe