Joachim said:It wouldn't be 30th level, more like 22nd... Apostle casts as Apostle level + 1/2 other casting levels. As you gain Mystic Theurge levels, you gain levels of Sorc/Wizard as well. For a Wiz(Sorc) 9 / Apostle 2 / Theurge 9 you would have 18 levels of Arcane caster and 20 levels of Divine caster.
Arcane - 9 + 9 = 18
Divine - 2 + 9 + 1/2(18) = 20
And you would have 9th level divine spells at 19th level and 9th level arcane spells at 20th level.
Nope, doesn't work that way. Your caster level does,indeed, get the bonus you describe. But that has no bearing on spells per day, or what spell levels you can or cannot cast. Per Table 5-2, that is determined solely by the Apostle's Class Levels. So you have to be an Apostle(9) to have access to 9th level divine spells, at all.
Further ... you do not get to double-dip the Theurge spellcasting increases, by counting it fully towards Apostle, and (via that caster level bonus) half of it again. That is a blatant attempt to abuse the rules.

Granted, because the Apostle is fast-tracked to 9th level spells, you do still have them in your example, but it does not work the way you describe. To correct your example of a Wiz(Sorc) 9 / Apostle 2 / Theurge 9:
Arcane: 9 + 9 = 18
Divine : 2 + 9 = 11; bonus of (9/2=) 4, for effective 15.