With the new clarification on SLAs allowing entry to prestige classes, does the following work:
Agathion-blooded aasimar (grants summon nature's ally ii, a 2nd-level Druid spell, therefore divine) sorcerer (any bloodline) 4, then go mystic theurge X to improve spellcasting as both a sorcerer and oracle without ever taking an oracle level, thereby gaining an oracle's spell power without ever having to select a curse or mystery?
Well if you're going to be using an aasimar, you could take Emberkin (or any other arcane ability types, or be a Tiefling) and a single level of Wood Oracle with the Bend the Grain power. That gets you into Mystic Theurge at level 4, with both casting types, depending on the feats and traits you take you can walk away with full CL on both sorcerer and oracle spells with only a 1 and 2 level lag in spell access. That setup is also PFS legal to my knowledge.
If you're worried about the negatives of the 'curse', certain ones are awesome, Haunted and Blackened add additional spells for minor penalties, Legalistic is fine as long as you keep your word, as you're a Charisma caster, Wasting provides a means to 'bow out' of being the party face and the impact of Wrecker is minimal to a spellcaster unless you use wands.
Echoing BigDTBone: Mystic Theurge adds to levels in a class you already have. In your example, you could only add levels to Sorcerer because you don't have a divine casting class.
When a new mystic theurge level is gained, the character gains new spells per day as if he had also gained a level in any one arcane spellcasting class he belonged to before he added the prestige class and any one divine spellcasting class he belonged to previously.