what Larian seem to have changed is a full caster class gives +1 to spells level... ie if you have 1 level Mage and 2 level cleric, you are seen as a 3rd level mage as for what spells you can cast and same for cleric... in essence what this does as long as you multiclass full spell caster classes you are a theurg...
This is speculation. I don't believe it's been verified at all.
