Just to go a bit more in the deails:
1.) You can't change the spellbook. So either Arnor and Gondor both have the army of the dead, or both don't have it.
2.) You can't give a faction as many heroes as you want, there is an upper border. That's also why the dwarven kingdoms have less champions than other factions and there are too many heroes for Gondor and Arnor together. (and basically your method is the same thing we did for the dwarves)