Okay I'm mixing things up a little because dangit why not you only live once.

There's a feat called Precocious Apprentice in Complete Arcane that grants you a single second level spell slot but only if you pick it at first level as an arcane caster (and there are some limitations included in the feat too). Then four levels of Favoured Soul gets you second level divine spell slots, meaning you qualify for Mystic Theurge at sixth level.

So my character will have the spellcasting of a fifth level Divine Soul and a second level Sorcerer and every time they level up they'll increase both. This is all purely mechanical, though. She's still going to be that heretical figure others believe stole powers from the gods, when she herself says she doesn't really know where this power came from.