This is just a Virtual Caster Level system. I've seen them before. They don't work.
Under the 3rd edition multiclassing system, a Wizard can afford to spend about 1 level picking up some other cool ability without falling too far below the power curve. Take more than that, and he essentially might as well not show up.
Under a VCL system, you still have the same problem. Once you've lost 2 caster levels, you are an entire spell level behind the curve. The number of levels you can afford to lose is still a finite number - it's just slightly larger.
Now you can take two Fighter levels instead of just one and still be in the game. Woo-hoo. Except of course, that if you split Fighter and Wizard you are still going to be obsolete. You'll do it at level 6 instead of level 4, but it still happens to you, so the VCL system hasn't actually solved anything.
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The basic problem is, that your number of Wizard Caster levels currently sets:
* The number of tricks you can do.
* The save DC of your only meaningful offensive abilities.
* Your ability to penetrate SR.
* The amount of damage your offensive abilities inflict.
* The duration for your tricks.
Now, the first one is kind of like a feat, a limited use ability, or any other class feature. When you take a level of Rogue you might get the "uncanny dodge" ability, and when you take a level of Wizard you might get the "web" ability.
But the rest of it is simply your ability to matter at your character level.
If your Caster Level is 11 or less when you fight a Pit Fiend, there is no chance whatsoever that you can affect it with spells.
When you fight a Roper, you had better be able to get a 30 (and thus, have a caster level of 10 or more) or your spells can't work.
And so on.
If your fireballs aren't scaling, you may as well not cast them. It would be like a Fighter not getting feats and weapon upgrades - the game just isn't designed for that.
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If you want multiclass wizards to be able to compete, you're going to have to remove all of the game mechanical directives which require them to have caster levels.
That means that save DCs can't be set by the highest spell level you can cast, nor by your Wizard level. I suggest 10 + 1/2 Hit Dice + Int Bonus. Just like the Wizard's spells were a Monster Ability.
That means that penetrating SR can't be set by your class level - it has to be set by your character level.
That means that your spell durations and damage dice can't be set by your class level, they have to be set by your character level.
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Once you've done that, of course, you don't really need a VCL system. When you take a level of Rogue you get a neat ability that you can use all the time, when you take a level of wizard you get some really nice abilities that can each be used only once per day.
-Frank