No need for a quote. Progressing another class's class feature, like MT does, does not constitute a class feature. There are a few PrCs which grant their own spellcasting (Assassin, for example, or Blackguard). These have their own spellcasting as a class feature. Mystic Theurge is different from these: it doesn't grant spellcasting itself, so there's nothing for Uncanny Trickster to progress. Mystic Theurge only references other classes which grant (arcane and divine) spellcasting, and progresses those class features.
The result is that Uncanny Trickster taken after MT could only progress actual class features granted by the class you want to progress with UT. MT doesn't have those (only progressing extraneous class featuers itself). So your UT levels could only be used to progress class features of another class you have, i.e. EITHER the arcane OR the divine caster you used to enter MT.
Furthermore, if you take MT to level 10, then take UT, you couldn't progress even any actual class features MT grants, because the class is already finished, and it doesn't have any abilities that read "...per class level" or some such (like Hellfire Warlock does, which is commonly progressed with UT).
Besides, MT has an epic progression.