Since spells scale with power level a multi-class spell caster is, and should be, less powerful than a single class spell caster.
However this does produce a pretty poor Wizard/Priest, Druid/Wizard, etc.
Solution: Add a passive +1 power level to all caster classes which stacks (Double casters will get this twice)
Pure caster = +3 power
Two Caster Class = +2 power
Melee/caster = +1 Power
This may be a problem with subclass wizards. The subclass penalty could also apply to druid/priest spells?