Epic Spellcasting and Mystic Theurge: how many spell slots?

Question

Assume an epic Mystic Theurge has picked up the Epic Spellcasting feat only once.

They can cast 9th level spells both arcane and divine.

They have 24 ranks in each of the relevant skills:

• Spellcraft
• Knowledge Arcana
• Knowledge Religion
• Knowledge Nature

The Epic Spellcasting feat:

Special: If the character meets more than one set of prerequisites, the limit on the number of spells he or she may cast per day is cumulative.

So does this theurge get six slots from just one allocation of this feat? Or does they have to buy it 3 times?

Answer 1

A caster needs the feat Epic Spellcasting only once

While the SRD on the feat Epic Spellcasting doesn't provide an example of how to apply the feat's benefits, the Epic Level Handbook does in the feat's Special entry:

For example, if you are a wizard/cleric, you may cast a number of arcane epic spells per day equal to your number of ranks in Knowledge (arcana), divided by ten, and a number of divine epic spells per day equal to your number of ranks in Knowledge (religion) or Knowledge (nature), divided by ten. (55)

So your mystic theurge is casting epic spells like a boss from just one iteration of the feat Epic Spellcasting. However, with 24 ranks in each Knowledge skill, he's only casting four—not six as the question posits—ultrapowerful gamewreckers per day. (You have to round those fractions all the way down.)

Also, unlike, for example, the feat Weapon Focus (PH 102), nothing in the Special entry of the feat Epic Spellcasting indicates it can be taken multiple times, so even if a caster wanted to take it multiple times—perhaps to cast even more epic spells per day?—, the caster couldn't.