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Spontaneous spellcasters can only select spells that they have prepared that day using non-spontaneous classes for this ability, even if the spells have already been cast. For example, a cleric/sorcerer/mystic theurge can use this ability to spontaneously cast a bless spell using a 2nd-level sorcerer spell slot, if the character had a prepared bless spell using a 1st-level cleric spell slot, even if that spell had already been cast that day.
If you want to cast a Sorcerer spell using a Cleric spell-slot you'd have to prepare it in the morning.
If you want to cast a Cleric spell using a Sorcerer spell-slot you don't have to prepare it, you can spontaneously cast it whenever you like during the day. The limiting factor is that you can only spontaneously cast the spells that you prepared using your cleric spell-slots. So if you have BESTOR CURSE and REMOVE CURSE prepared in your 3rd level cleric spell-slots for the day then you can spontaneously cast either of those spells as sorcerer spells using 4th level sorcerer spell-slots (assuming you're a 5th level or higher Mystic Theurge).
Actually reading it closely, it would appear that you can use your sorcerer spell slots for your prepared cleric spells, but not the other way around... it says they can ONLY select spells prepared from their non-spontaneous casting class for this ability.
Click the link, I didn't quote the entire thing.
You can prepare a 1st pevel sorcerer spell in a 2nd level cleric spell-slot.