FWIW I think the MT is a poor solution to the actual problem - how do you multiclass spellcasters without them getting completely gimped.
I've actually come up with a simple game mechanics solution that is, to my mind, far more useful, scaleable and broadly applicable.
You can read the detailed discussion about it in this house rules thread here
http://www.enworld.org/showthread.php?t=166497
Here is the gist of it:
1. Caster level determines spells known and not just spell characteristics.
2. Caster level in any class = that classes caster level + half all other class levels (rounded down). Thus whether Sor10/Wiz10, Wiz10/Clr10, Clr10/Dru10, you have caster level 15 in each of your classes and have spells known and castable as per that. Note that for Clr10/Wiz10 this exactly mirrors what you would get with Clr5/Wiz5/Mystic Theurge10.
3. To avoid problems with grossly unequal levels (e.g. 18th level fighter taking 1 level Wizard ), your caster level can be no more than 50% more than your casting class level.
That leaves the 10/10 multiclass in the position that works OK, it stops the 18/1 multiclass from getting absurd and gives the following breakpoints (using ftr/wiz as an example).
Wiz20 = CL20
Ftr2/Wiz18 = CL19
Ftr4/Wiz16 = CL18
Ftr6/Wiz14 = CL17
Ftr8/Wiz12 = CL16
Ftr10/Wiz10 = CL15
Ftr11/Wiz9 = CL14
Ftr12/Wiz8 = CL12
Ftr13/Wiz7 = CL10
Ftr14/Wiz6 = CL9
Ftr16/Wiz4 = CL6
Ftr18/Wiz2 = CL3
Ftr19/Wiz1 = CL1
Ftr20 = CL0
For those who don't think there is a problem with caster multiclassing, move on, nothing to see here
For anyone who would like to try a non-prestige class, works from ground up solution, this one might be worth thinking about.
Cheers