3E multiclassing does allow character abilities to stack, in general. Caster level, though, can't easily be one of these. If you MC a caster/non-caster, the non-caster has no caster levels to contribute. If you MC a caster/caster each has it's own spell list, spells/day and caster level.
To retain balance and allow a caster/non-caster to stack levels, non-casters would have to be given caster levels that have some meaningful effect for them - for instance you could re-work the magic system so that spellcasting requires an opposed caster level check to affect someone...
To retain balance and allow a caster/caster to stack levels, you'd have to, at least, consolidate thier spells/day...
Either would be a major change to the rules.
PrCs can be one way to address the issue. For instance, instead of being a Fighter/Wizard your whole carreer you could change to Bladesinger or Spellsword. Instead of evenly advancing as a Sorcerer/Druid you could change to Geomancer.
I also have a minor 'solution' I'm considering for caster/caster combinations:
Blended Technique [Metamagic]
    You combine different spellcasting techniques to make a spell more potent.
    Prerequisites: Ability to cast the same spell from two different lists (Wizard & Sorcerer lists count as 'different' for this purpose).
    Benefits: When you use a spell with this metamagic feat as part of your daily spells for two (or more) caster classes you have, you add your levels in those classes together to determine your caster level for that spell. A Blended spell takes up a slot of the same level as the spell normally does, but it takes up a slot from each class involved, and both are used when the spell is cast. Casting a Blended spell does not take extra time, though if one of the classes involved is casting spontaneously, the usual time increase aplies.
EX: A Cleric 5/Wiz 5 has Dispel Magic in his spellbook. This spell is also available as Cleric spell. He chooses to prepare Blended Dispel Magic, taking up one third level Cleric spell slot and one third level Wizard spell slot. When he casts Dispel Magic, both spell slots are used up, but the Dispel functions as if he were from a 10th level caster.
EX: A Wiz 5/Sor 6 has Fireball in his Spellbook. He chooses to prepare Blended Fireball in both of his 3rd level Wiz slots. If he wishes to cast one of these fireballs, he must also spontaneously 'Blend,' a Fireball as a Sorcerer, taking a FRA to cast, and using one 3rd level spell for the day as a Sorcerer and one of the prepared Blended Fireballs, but inflicting 10d6 damage (as an 11th level caster). The character could also use regular spontaneous fireballs, inflicting 6d6, as normal. If he were to use all his 3rd level spells for the day as a Sorcerer, the prepared Blended Fireballs would be useless, as there is nothing to combine them with.