I think I like the Arcana Evolved system of combined spellcasting a little better (add together spell slots and caster levels, use either casting stat); this system seems to encourage multiclassing a lot (i.e. a wizard would seem to sacrifice very little by taking a few levels of cleric).
Wulf's is much more elegant, but I came up with a simple idea if multi-classing casters is an issue for you:
Simply make "+1 level of arcane spellcasting class spells" and "+1 level of divine spellcasting class spells" feats (with a caveat that this feat cannot raise your caster level past your character level - 2.) (Thus, you couldn't be a Wizard 17/Fighter 3 and have an effective Wizard level beyond 18.)
If a Cleric 1/Wizard 19 maxed out their 10-12 feats (Pathfinder), they could essentially be a Wizard 19/Cleric 13 for spellcasting prowess. That seems reasonable given the feat sacrifice.
Or, if it seems unfair to require so many feats, use something like...
Divine Affinity (or whatever)
Prerequisite: Cast 1st level divine spells
Benefit: You gain +1 spellcasting level of your chosen divine spellcasting class spells for every two levels taken in non-divine spellcasting classes. Your divine classes stack for the purposes of determining caster level.