Look at the trait Magical Lineage. Say at level 1 the character took a levels in either wizard or cleric. For the trait Magical Lineage, the character picks a spell that appears on both the cleric and wizard spell lists, like summon monster I. (The trait Magical Lineage is exclusive to a single spell, not to a specific class.)
At level 1 the character also takes the feat Heighten Spell. At levels 2 and 3 the character takes either a wizard level then a cleric level or vice versa. And, by level 3, the character has 3 skill ranks in both Knowledge (arcana) and Knowledge (religion).
So at level 3 by virtue of the feat Heighten Spell and trait Magical Lineage the character's able to cast a 2nd-level summon monster I spell as both an arcane spell and a divine spell. This lets the character take at character level 4 level 1 of the prestige class mystic theurge.
Does this work this way?