Ok... how about this.
When you cast a combined spell there is a chance of outright failure. This chance is calculated in the following way. Base chance 50%. Plus 10% per spell level of the combined spell (2 MM = +20%) minus 5% per caster level.
Thus a level 3 wiz casting a combined MM would have a 50(base)+20(2 spell levels)-15(caster level) = 55% chance of spell failure due to the complex requirements of casting 2 spells at once.
A level 20 wizard casting a combined Stoneskin and fire shield (as a level 8spell) would have a 50+80-100= 30% chance of spell failure. Conversely, the same wizard could cast a combined fireball and hold person (as a level 6 spell) and have a 50+60-100=10% chance of failure.
