What Prestige Class to finish this out?

[Yogibear41]

I would like a dual progression prestige class that I can use with:

Wizard1/Archivst3/Mystic Theurge10/X6

So I can have 19th level Archivst Casting and 17th level wizard casting, at level 20.

[Phelix-Mu]

Find an efficient way to get the Trackless Step class feature in three levels or less (three for both scout and druid, and those are the only ones a brief bit of book-skimming turned up) and you can get into Arcane Hierophant, which is otherwise just better than MT in the first place.

Another trick I hear mentioned is Legacy Champion, but I don't know if it works for spellcasting progression.

[dextercorvia]

The best I can offer is Wizard1/Archivist2/MT6/Wildrunner1/ArcaneHierophant10

That gives you 17/18 Wizard/Archivist casting.

You have to use Early entry on both sides, and be an Elf or Half-Elf to make it work. I recommend Metamagic School Focus and Heighten Spell, but there are plenty of other ways.

*You'll also, really want a way to get Survival as a class skill.

[Talionis]

Find an efficient way to get the Trackless Step class feature in three levels or less (three for both scout and druid, and those are the only ones a brief bit of book-skimming turned up) and you can get into Arcane Hierophant, which is otherwise just better than MT in the first place.

Another trick I hear mentioned is Legacy Champion, but I don't know if it works for spellcasting progression.

I think Legacy Champion works. It doesn't work for classes like Chameleon or Soldier of Light where the progression ends on a table or has no suggestion that the class would ever get higher level spells than level 6. Now you do loose at least one caster level, but being able to advance both casting is pretty darn powerful.

[Spore]

Ask your DM to extend the PrC

[Phelix-Mu]

Ask your DM to extend the PrC

The feasibility of this, while an excellent and simple suggestion, depends heavily on the DM's view (something that many want to restrict the influence of in regards to their character build, even if that isn't strictly possible). Many DMs might sensibly think that the cost of having dual, full-casting progression is that you don't get dual 9ths, even if optimization easily points out that dual-casting is not worth the cost outside of early entry stuff, which can, in many cases, eke out dual 9ths.

Legacy Champion may also be above such a DMs cheese tolerance. I'd go for the rather flavorful MT/Arcane Hierophant build above, but if this character is already in situ, so to speak, that might not be an option (quite apart from the racial restriction of Wildrunner curtailing design options rather seriously).

The only other divine/arcane dual-progression that pops to mind is True Necromancer, and that isn't even 100% full progression, not to mention even more flavor-restrictive than Arcane Hierophant.