I have a player who has a 5th level Druid/3rd level Catlord. The Catlord is allowed cat companion animal(s) of up to his hit dice x 2, even if he's an adventurer. Would this stack with the druid's animal companion abilities (thus allowing an 8 HD dire tiger or even an 11 hit die dire tiger that has been Advanced), or should each pool of animal companions be treated separately?