As far as i know, you will get more cantrips... but spell slots is shared between all classes... 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...
so we will have access and know spells faster and earlier... but we will not get more spells slot to cast these... its not as bad some make it out to be, it would be worse if casters got more spell slots...
