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.
Most of my question is in the verbiage of the first sentence. Are the two spellcasting classes mentioned restricted to the ones being leveled up by the prestige class? Or would combined spells allow a character that has also dipped a level in a occult casting class to cast one of their level 1 spells using one of their level 2 arcane or divine slots?