What are all the dual progression prestige classes

RAW: Ambiguous

The rules are not clear on what, exactly, constitutes an “arcane spellcasting class” or a “divine spellcasting class,” probably because initially it was obvious. By default, assassins, bards, sorcerers, and wizards were the arcane spellcasting classes, blackguards, clerics, druids, and paladins were the divine spellcasting classes. There was no ambiguity: the former are the classes that cast arcane spells, and the latter are the classes that cast divine spells. Easy.

Then they printed things like Alternate Source Spell, Rainbow Servant, Sha’ir, and Southern Magician, which introduced ambiguity that hadn’t existed before. In these cases, you have spell slots from one class being used for either arcane or divine spells.

Does this new feature suddenly make the class into an “arcane spellcasting class” or “divine spellcasting class” where it wasn’t before? There hadn’t ever been a strict definition before, and they didn’t print one at this point, either. Some of these effects seem to try to include wording that prevents this kind of thing, but much of those rules are also unclear. For instance, consider this from Southern Magician: “The actual source of the spell's power doesn't change,” which Customer Service interpreted as preventing entry to mystic theurge. But it doesn’t really say that, does it? It says something about power source, which is unclear.

Unfortunately, there’s no direct, rules-as-written, “as it says on page xyz of Complete Shenanigans” kind of answer to this question.

Recommendation: Never

RAW is ambiguous, but what’s going to work well in-game is not: never, under any circumstances, should one be allowed to advance wizard spellcasting faster than the wizard does. That should never, ever happen in any game, and if you’re going to allow it you might as well allow Pun-pun.

Allowing these sorts of tricks to qualify for mystic theurge, and other prestige classes and feats that require one type of spellcasting or the other, is pretty clearly legal, RAW, and also usually far less troublesome. The only exception I’d be likely to make is the dweormerkeeper from Complete Divine’s web enhancement, but then I’d probably just ban that class outright.

Even allowing a divine-only prestige class to progress wizard spellcasting is almost-always not a problem. It’s the double-progression that should never, ever happen.

No, or at least pretty much everyone agrees you can’t

As Thomas Jacobs says, there is no hard-and-fast definition given by the books for “spellcasting class.” Various non-book sources (FAQ, Customer Service, and so on) have repeatedly denied the ability to advance mystic theurge with another prestige class that advances a “spellcasting class,”¹ but such sources are not officially a part of the rules (per errata documents).²

That said, a community consensus does exist,³ and it is just about universal:

A class that advances spellcasting is not, itself, a spellcasting class; a spellcasting class is one that has its own spells.

So, with that in mind:

• assassin, bard, blackguard, cleric, druid, paladin, ranger, sorcerer, and wizard are the core spellcasting classes, even though some of them are prestige classes.

• arcane archer, arcane trickster, archmage, dragon disciple, eldritch knight, hierophant, lore master, mystic theurge, and thaumaturgist are not, even though they require or advance spellcasting.

The classes in the first list could all be advanced by mystic theurge, and the arcane ones (assassin, bard, sorcerer, wizard) could be advanced by anima mage, but the classes in the second list cannot be advanced by anima mage, even the ones that advance arcane magic.

There are two prestige classes, legacy champion in Weapons of Legacy and uncanny trickster in Complete Scoundrel, who can advance any other class. These prestige classes could advance mystic theurge and therefore advance two other spellcasting classes, one arcane and the other divine.

1. ^{I cannot find the precise location of these mentions, but I do find reference to it (and to the fact that this stuff keeps getting deleted).}

   ^{As StevenO mentioned, we hashed this argument out ages and ages ago (circa 2004, I think - as soon as the Mystic Theurge was previewed pre-3.5, in fact, as it was the first dual-caster out there). However, most of the threads discussing this were destroyed in assorted board purges, and I don't see the discussion in the official FAQ either. We did get a Sage ruling on this and, unusually, a unanimous response from customer service (believe me, that is bizarre!), along with a solid consensus from die-hard rules lawyers who were more adept with mathematical logic than I (see, for instance, our threads discussing the world damage record, most of LordofProcrastination's dirty tricks, or any serious thread discussing either Pun-Pun or his challengers, which frequently delve into cardinality, infinity, and philosophy of time as well).}

2. ^{You can find all errata for 3.5 on Wizards’ official page for 3.5 updates; each errata file opens with a section explaining how errata works and defining primary and secondary sources. At best, the FAQ and CustServ are secondary to literally everything else, but most don’t grant them even that title. See here for more on what is wrong with the 3.5 FAQ.}

3. ^{Quick examples of community consensus:
   • Here’s one thread, with one voice of disagreement
   • A second thread with no disagreement, but an unanswered call for evidence
   • Another thread with no disagreement (and an alternate interpretation of why this doesn’t work)
   • Plus I managed to find one from Wizards.com, rather difficult these days (scroll down, it’s the last four posts of the thread).
   • The Mystic Theurge Optimization Handbook makes no mention of the possibility, but lists “No Special Abilities for advancing levels” as a disadvantage; this would not be an issue if you could just progress mystic theurge with some other prestige class. It also does not use such an option in any of its example builds, which it doubtlessly would if it were considered valid.
   Note in all cases that this is taken as obvious; the consensus is really strong and most don’t realize there is any ambiguity about it.}