Be an Illumian with any Krau sigil combination. That gives you +2 CL to all casting classes, not to exceed your HD. Then start Beguiler 1, and take 4 levels of Conjourer. Your 1st level feat should probably be Able Learner (Illumians are Humanoid(Human), so they qualify), your 3rd level feat should be Practiced Spellcaster (Beguiler). That, combined with the Illumian racial feature will keep your CL even throughout your whole progression.
Now when you take Ultimate Magus, and you have one of the 3 levels where you progress only one side, you pick the side with the lower CL. Since both CLs are equal, you get to choose where to put it. Always put it on your Wizard side. The Practiced Spellcaster + Illumian trait will keep the CLs equal.
The end result is 19 levels of Wizard progression and 8 levels of Beguiler progression instead of something dumb like 16 levels of Wizard progression and 11 levels of Beguiler progression. Higher level spells are better than lower level spells. Keep your Wizard casting as pure as possible.
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