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.

Assuming the arcane spellcasting class has a poor Fortitude save and Bluff as a cross-class skill, you need a single level of anything that has a good Fortitude save and Bluff in-class. Then the +1 of your arcane class’s poor Fortitude save after 3 levels will add with the +2 of the good Fortitude save from the new class, and you can just dump all your skill points in Bluff (ideally, a high-skill class makes this easier, though if your arcane class is intelligence-based it may not matter).

You still need 5 overall levels to get the Knowledge (religion) and Spellcraft ranks, however, so that is a hard limit. You thus want 4 levels in whatever your arcane spellcasting class is, plus one level of something else.

Many classes fit this description, but none are likely to work as well for you as savage bard:

• Good Fortitude save

• Bluff, Knowledge (religion), and Spellcraft in-class; no need for any cross-class ranks

• 6 + Int skill points, making it trivial to get the 6 you need in Bluff

• Good 1st-level class features overall

  • Bardic Music means you can qualify for sublime chord, an arcane prestige class similar to ur-priest in its quick progression. You’d still (just barely) get 9ths from your typical prepared arcane spellcasting class, so this is kind of unnecessary, but it’s a neat alternative, and it does hit 9th-level spells a character level sooner.

The order of your savage bard levels and levels in the other class is going to have to be a calculation on your part. If you take savage bard first, you get to multiply its base 6 skill points by ×4, which is probably better than your other spellcasting class gets. However, you’re also limited to 4 ranks in any one skill, which means you are going to have to put cross-class ranks in Bluff to get the remaining 2 ranks – costing you two more skill points than it would otherwise. Mathematically, savage bard first is better (6 additional skill points if your other class has 4+Int skills, 14 additional skill points if your other class has 2+Int skills), but you’ll have to make sure you can get all the ranks you need with your lower-skill-point-per-level class having to pay cross-class prices. Especially if that class does not get Knowledge (religion) in-class.

Because bard is Charisma-based (as is sublime chord, if you go that route), and you only barely get 9th-level spells from a prepared arcane spellcasting class (and wouldn’t from a spontaneous one), sha’ir from Dragon Compendium might be a better choice than wizard or sorcerer. It has you prepare spells (sort of), and thus gets new spell levels “on time” unlike the sorcerer, but it’s also Charisma-based. It does get a small amount divine spellcasting, though, weirdly enough: ur-priest requires that you forsake this ability entirely, but it’s no great loss.