No.

Character Advancement
When adding new levels of an existing class or adding levels of a new class (see Multiclassing, below), make sure to take the following steps in order. First, select your new class level. You must be able to qualify for this level before any of the following adjustments are made. Second, apply any ability score increases due to gaining a level. Third, integrate all of the level's class abilities and then roll for additional hit points. Finally, add new skills and feats.

The order of how you level matters. You're allowed take a feat the same level you gain the prerequisites only because gaining feats is the last thing you do. If the prerequisite is a skill, spell, or ability, you already have it by the time you take the feat.

Of course, if the prerequisite is another feat, you must already have the prerequisite feat before you can take the desired feat. However, since character advancement is an ordered process, if you gain multiple feats in a level¹, you can choose to take the prerequisite feat first. You can now take the desired feat.

In your example, you can take neither feat first since neither feat has its prerequisite met.

¹ Class abilities that give bonus feats give them at the time you normally add new feats (after skill points). It's unclear if you choose your standard feat before your bonus feat or vice versa, but I would generally allow players to select feats in any order if it matters.

This is undefined under the rules

Unfortunately, the rules don’t explicitly say.

• Some argue that you simply cannot choose that class again, and therefore lose that benefit unless you have some other class to choose

• Some argue that you should look for epic progression for class you’ve finished, and barring that should make one

• Some think you just get as much as you can (improved Caster Level), and ignore the rest (additional/better spells per day/spells known)

Looking at the strict RAW, the first point seems to me to be mostly wishful thinking; the rules don’t ever actually say that you cannot choose a class that’s “finished,” and for that matter they don’t exactly even make it clear that you cannot take more levels than are listed in the table (look up some of the arguments about combining Hellfire Warlock and Legacy Champion, where this is hashed out in great detail).

Thus, the latter two seem more likely. I don’t like the epic rules and don’t think it’s a good idea to give out those effects early (or at all <.<), so I favor the last point. Furthermore, the special requirements of epic prestige classes lead me to believe you cannot get them before 21st level, no matter what you do. And unless the Epic Level Handbook is explicitly in play, those rules may not exist at all, so there may not be such a thing as an epic anything.

Note that, even if you do get the epic progression, it very likely won’t actually give you anything aside from improved caster level, since most epic progressions don’t and instead use bonus feats to grant you improved spellcasting. Even epic bonus feats require you to meet requirements, and that includes being an Epic character which you are not.

Anyway, ask your DM. Since your DM has already allowed Wizard 5/Ur-priest 2/Mystic Theurge 8, even though you don’t actually legally qualify, he’s clearly a very different DM than I, or any I have ever played under, so maybe he’ll let you get 10th level spells before you could ordinarily get 9ths. Why he’d allow that, I have no idea; I certainly wouldn’t. But based on what he’s already allowed, he just might.