The iteration I have suggested in QC is the following (this is only my suggestion and has received no comments from anyone else in QC, therefore it is unlikely to be what a final implementation would look like, were the class to be approved and added to the server).
Requirements
Skills: Knowledge (arcana) 6 ranks , Knowledge (religion) 6 ranks
Spells: Able to cast 2nd-level divine spells and 2nd-level arcane spells.
Class Restriction: Wizard only (arcane); Cleric or Druid only (Divine), may not have any other base class.
PRC Restriction: May not have any other PRC. (so only builds is W/C/MT or W/D/MT)
Deity Restriction: Must worship a Deity with the Magic Domain. (Akadi, Azuth, Leira, Lliira, Mystra, Siamorphe, Laduguer, Kiaransalee, Henali Celanil, Sehanine Moonbow, Baravar Cloakshadow, Segojan Earthcaller, or Brandobaris)
**Deity restriction may need to be expanded for consideration of Druid MT's**
***Divine Power is disabled / cannot be cast by a character with the Mystic Theurge PRC***
Class Features:
BAB: Low
Saves: High Will, low fort/reflex
Spell Casting Progression: 9/10 in primary, 6/10 in secondary (I think I had level 5 or 6 as the 'dead' level for the primary and levels 2, 4, 7, and 9 for the secondary.)
Why did I set it up like this? Well, looking at a few different build options shows the following:
1. W17/C3/MT10 you could have CL30 Wizard with PSC and CL: 9 (13 w/ PSC) for Cleric (or Druid).
2. W10/C10/MT10 would give CL 23 for the primary and CL20 for the secondary.
3. W11/C9/MT10 where Cleric is selected as the 'primary' casting class. This would give CL17 in Wizard, CL18 in Cleric (21/22 with PSC).
So by balancing it in this way, you can reach exactly CL30 in your primary casting class by taking Practiced Caster and limiting your secondary casting class to 3 levels, but you also can just barely reach the ability to cast level 9 spells in both classes, though you have to deal with having minimal CL in both as well.
As I said above, I just recently posted this and there has been no QC discussion around it. Even if there is I'm pretty sure leveling/progressing two spell casting classes is probably pretty complicated/difficult to build and implement so would take some time. In any case, this should ONLY be taken as my opinion, and not that of the staff or management of BGtSCC
Anyway, enjoy ripping it to shreds for being underpowered or what not.