Ok, taking a few steps back to clarify:
When you decide to focus on MT, you are going for a wide variety of spells at the cost of delaying access to high level spells.
You can probably get into the class at lvl 8 or earlier.
Let's assume you enter at lvl 8.
So, when you reach lvl 18, you have the following options:
1. pick up levels from your original two classes (one at a time)
2. try to find another arcane/divine dual progression prestige class
3. take some other classes until you reach lvl 21 and can take epic levels in MT
Since I would prefer option 2, I suddenly realised that this was rather silly if the same could be accomplished by just expanding the existing PrC.
Hence my question: what would be the balance issue?
Herzog
ps: Please don't start about the Epic Level MT. I know it sucks.