I've dumped the old version of the Theurgist feat (above), and instead have built a new concept inspired by Cheiromancer's thread....
http://www.enworld.org/showthread.php?t=135992
Here's his main "theurgist" feat, modified for my campaign, and with higher Prerequisites...
Theurgic Adept [General]
By Cheiromancer – modified for World of Kulan
Your arcane and divine caster levels stack when determining caster level.
Prerequisites: Int 15, Wis 15, Cha 15, Knowledge (arcana) 6 ranks, Knowledge (religion or nature) 6 ranks, ability to cast both arcane and divine spells, Mystic Insight* or Insight into the Green*.
Benefit: Your arcane and divine class levels stack (to a maximum of your character level) for the purpose of determining your caster level. This does not affect your spells known or your spells per day.
*See the linked thread for these feats.
And here are my creations, in homage...
Theurgic Savant [General] (NEW)
You gain a bonus on your theurgic arcane and divine spells' save DCs.
Prerequisites: Int 17, Wis 17, Cha 17, Knowledge (arcana) 8 ranks, Knowledge (religion or nature) 8 ranks, ability to cast both arcane and divine spells, Mystic Insight or Insight into the Green, Theurgic Adept.
Benefit: You gain a +2 insight bonus on all save DCs for your chosen theurgic arcane and divine classes' spells.
True Theurgist [Epic] (NEW)
Your arcane and divine caster levels stack when determining number of spells.
Prerequisites: Int 19, Wis 19, Cha 19, Knowledge (arcana) 20 ranks, Knowledge (religion or nature) 20 ranks, ability to cast both arcane and divine spells, Mystic Insight or Insight into the Green, Theurgic Adept, Theurgic Savant.
Benefit: Your arcane and divine class levels stack (to a maximum of your character level) for the purpose of determining your normal spells known and your normal spells per day, but not for any epic spells.
Epic Theurgist [Epic] (NEW)
Your Knowledge skill ranks stack when determining number of epic spells.
Prerequisites: Int 21, Wis 21, Cha 21, Knowledge (arcana) 25 ranks, Knowledge (religion or nature) 25 ranks, ability to cast both arcane and divine epic spells, Mystic Insight or Insight into the Green, Theurgic Adept, Theurgic Savant, True Theurgist, Epic Spellcasting.
Benefit: Your Knowledge (arcana) and Knowledge (religion or nature) ranks stack (to a maximum of 100 ranks) for the purpose of determining your epic spells per day.