What has CON got to do with it?
Here's another possible way of aproaching the problem:
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.