Simple question: what Arcane PrCls advance the "spells" of 2 arcane classes simultaneously? I know there is one that advances a spontaneous caster and a "memorizer" simultaneously- the Archmage?- but are there others?
Complete Mage p.77- Ultimate Magus.
This is the only PrC I know of that does what you are asking.
In a similar flavor, Expanded Psionics Handbook p.141 has Cerebromancer, progressing Psionic class and Arcane class, pulling off something similar with Psion/Wizard.
Complete Mage p.58 has the Eldrich Thurge, progressing Warlock and Wizard, also a similar flavor but not exactly what yoiu're asking.
Levels of prestige classes that provide a +1 level of spellcasting effectively stack with the warlock's level to determine his eldricth blast damage (treat his combined caster level as his warlock class level when looking at Table 1-1: The Warlock to determine eldritch blast damage) and his eldritch blast caster level (half his total caster level from his warlock levels and his levels in a prestige class that grant him an increased spellcasting level).
“An eldritch blast is the equivalent of a 1st-level spell. If you apply a blast shape or eldritch essence invocation to your eldritch blast (see page 130),
your eldritch blast uses the level equivalent of the shape or essence.”
Any other references to eldritch blast being something other than the equivalent of a 1st-level spell should be disregarded.