I went ahead and developed a Mystic Theurge prestige chain that allowed for a little diversity for your casting classes (just as the Mystic Theurge does).
Mystic Theurge Training [Prestige]
Prerequisites: No other prestige feats, Spellcraft 6 ranks, Knowledge (Religion) 6 ranks, Ability to cast 2nd level Arcane and Divine spells
Benefits: The following skills become class skills for you: Concentration (Con), Craft (Int), Decipher Script (Int), Knowledge (arcana) (Int), Knowledge (religion) (Int), Profession (Wis), Sense Motive (Wis), and Spellcraft (Int).
In addition, choose one arcane and one divine Spellcasting class in which you can cast 2nd level spells. These classes are treated as having +1 caster level for every 2 Prestige feats you have (including this one).
Theurgic Stamina [Prestige]
Prerequisites: Mystic Theurge Training, no more than 3 feats listing Mystic Theurge as a prerequisite
Benefits: You gain the benefit of Expanded Spell Stamina for one arcane and one divine Spellcasting class in which you can cast 2nd level spells.
You may take this feat multiple times. It’s effects stack.
Theurgic Knowledge [Prestige]
Prerequisites: Mystic Theurge Training, no more than 3 feats listing Mystic Theurge as a prerequisite
Benefits: You gain the benefit of Expanded Spell Knowledge for one arcane and one divine Spellcasting class in which you can cast 2nd level spells.
You may take this feat multiple times. It’s effects stack.
Spellcasting Pinnacle [Prestige]
Prerequisites: Any 4 feats listing Mystic Theurge Training as a prerequisite
Benefits: Choose a single 3rd level arcane spell and a single 3rd level divine pell available to your Spellcasting classes capable of casting 2nd level spells. You may cast each spell once per day with a caster level equal to its associated class’s caster level.