I'm looking at possibly going mystic theurge with one of my characters, but I have a question about oposition schools and spell slot usage.
If I prepare a divine spell from one of my oposition schools in one of my arcane slots, would I have to use two spell slots one level higher than the spell I'm preparing?
If I prepare an arcane spell from one of my opposition schools in a divine slot, would I have to use two slots then?
I'm not sure how these would interact, I'm thinking it's a property of my arcane spells, rather than divine, but this could allow some cheese of using a higher level spell slot, instead of using two spell slots for spells on both lists.
Well, the way I read it, it breaks down to:
1) Arcane spell takes an additional slot.
2) Making it Divine increases required spell level by 1.
Since BOTH slots would be turning into divine, you'd need to increase BOTH slots by 1. So a 1st level Arcane would be 2 2nd level divine.
DM interpretation is always key, just present the argument to your DM, and see if they'll roll with how you'd like it!
Combined Spells (Su)
"A mystic theurge can prepare and cast spells from one of his spellcasting classes using the available slots from any of his other spellcasting classes. Spells prepared or cast in this way take up a slot one level higher than they originally occupied. This ability cannot be used to cast a spell at a lower level if that spell exists on both spell lists. At 1st level, a mystic theurge can prepare 1st-level spells from one of his spellcasting classes using the 2nd-level slots of the other spellcasting class. Every two levels thereafter, the level of spells that can be cast in this way increases by one, to a maximum of 5th-level spells at 9th level (these spells would take up 6th-level spell slots). The components of these spells do not change, but they otherwise follow the rules for the spellcasting class used to cast the spell."
The final sentence appears to be critical here: you use the rules for the class used to actually cast the spell. Therefore, a wizard with Evocation as opposition could use a single cleric 4th level slot to cast Fireball.
And if Conjuration were in opposition, you'd need 2 second level wizard slots to prepare Cure Light Wounds.
However, I'd recommend not playing a Mystic Theurge unless your group is using the optional guild training rules (specifically Eclectic Training and Esoteric Training) to regain casting levels. Or use Kobold Press' Theurge class (Theurge).
I'll have to talk to my DM, because that last line would imply to me that the rules are based on the class the spell came from not the spell slots used. However, Gilarius could very well be correct.