More than 6 hours with no reply, not too common around here. Go figure I ask a question that people don't wanna touch with a 60' pole. *sigh* I shall rephrase my question into an open debate.
Upon research, I don't blame people for overlooking this thread. This is a question that muddles the line between rules as written and rules as interpreted. The PHB entry for Animate Dead says "No matter how many times you use this spell, however, you can control only 4 HD worth of undead creatures per caster level." If Animate Dead were to be kind and say per hit dice, or per class levels, or per highest arcane/divine class level with access to Animate Dead, then there would be no debate. However, they chose to say per caster level. Caster level is usually class dependent. If I cast fireball using a sorcerer spell slot of a Wizard 10/Sorcerer 9, I don't consider the wizard's caster level whatsoever, even if he knows fireball, too. When I cast as a Wizard, it's 10d6, when I cast as a Sorcerer, it's 9d6. Sorcerer gets no penalty for Wizard knowing Fireball and visa versa because their spells are independent of each other.
It's not fair to compare Animate Dead to Fireball, however. The effects of Fireball don't accumulate indefinitely with every casting. Perhaps a Delayed Fireball with Time Stop, but not the way Animate Dead is designed. However, there is no getting around the question of "How?" Animate Dead is among the few spells which caster level functions outside of it's effect. I've looked in both the Caster Level section and the Multiclass section to find little-to-no relevant information on if something that normally has no opportunity to stack can stack given rare opportunity.
The only place which suggests that this couldn't stack is in the Combining Magical Effects section:
Player's Handbook said:
Same Effect More than Once in Different Strengths: In cases when two or more identical spells are operating in the same area or on the same target, but at different strengths, only the best one applies. For example, if a character takes a –4 penalty to Strength from a ray of enfeeblement spell and then receives a second ray of enfeeblement spell that applies a –6 penalty, he or she takes only the –6 penalty. Both spells are still operating on the character, however. If one ray of enfeeblement spell is dispelled or its duration runs out, the other spell remains in effect, assuming that its duration has not yet expired.
If max controllable undead can be compared to duration, or Animate Dead being a spell that affects the user rather than the animated corpses, then the highest max controllable undead would be the only one that counts. It's so confusing! Somebody give some input before I cry....