Okay, I think I understand now, but I have another question. To continue the example, let's say the theurge is now a Cleric 10/Wizard 10/MT 10. His caster level is 20 for both cleric and wizard. Does he now have to take 10 more 'regular' levels in either cleric or wizard and reach, say, 21st Cleric level before he's considered an Epic Cleric, with access to the bonus feats etc? Or can he somehow shortstep that and be considered an Epic Cleric at 11th Cleric level, because he's got his 20 casting levels?
Edited for clarity.