Ultimate Magus

[saggitar]

Would you be able to advance Ultimate Magus casting with classes that advance casting "as if you had also gained a level in an arcane spellcasting class to which you belonged before adding the prestige class level"?

Aside from being broken and therefore the danger of "DMG to your head" - why wouldn't it be possible?

[TenaciousJ]

Ultimate Magus doesn't have its own casting, so you can't progress it with another prestige class that advances casting in 1 class.  You would need another class that has the ability to progress the 2 base casters you're using.