Caster Levels Progression Variant (3.5e Variant Rule)

Introduction
Guess why spellcaster don't multiclass, why the Mystic Theurge sucks, why your True Necromancer sucks at necromancy... They all lost caster levels; this variant is made to fix this problem. This variant focuses on multiclassing with other spellcasting classes, however there is no shame to apply it to other classes as well. Never again shall your Gish suck (they won't equal a full spellcaster jfi).

Mechanic
Each levels of a spellcasting class (like sorcerer) stack with other spellcasting class for determining caster level, spell known and spell per day. However each spellcasting level of class X only count as 1/2 caster level for class Y rounded down (minimum of 0). Here an example: Magie is a level 3 conjuror and a level 4 beguilder. She cast spells like a level 5 conjuror and as a level 5 beguilder. She however do not gain any class feature, bab or save of the classes extra level (Thus she wouldn't have a bonus feat, the 5th level wizard class feature).

Spellcasting advancing prestige classes are slightly different. Since they effectively stack with the advanced spellcasting, they do not provide tis bonus to the advancing class. However they do provide bonus to classes not advanced in spellcasting. Here an example, Machoor is a level 4 wizard and a level 5 cleric, she also have 10 Mystic Theurge levels. She cast spell as a level 17 cleric and wizard spells as a level 16 wizard. If she had a single sorcerer level she however gain a bonus to it, she would cast spells as a level 10 sorcerer(+2 from cleric, +2 from wizard +5 from Mystic Theurge and +1 for her actual sorcerer level).

Spellcasting PrC with non-full casting progression are affected by this variant, but only count levels without spellcasting progression.

Some DM may feel like martial classes should stack the same way. There is no unbalance using such system on classes in general.

Creator Note
It is greatly suggested to increase the spellcasting requirement for classes such as the Mystic Theurge or the Ultimate Magus. Since with this varaint it is possible to create unbalance using these classes.