Larian hasn't given specifics, aside from [paraphrased] "we want multiclassed spellcasters to get access to higher level spells (like fireball) earlier."
Do you treat your total level as your spellcasting class level for spells known? I.e., a 10 Barbarian/1 Wizard can learn 6th level wizard spells.
Do you only add up your "caster class" levels? I.e., a 10 Barbarian/1 Wizard can only learn 1st level spells. But a level 10 Cleric/level 1 Wizard can learn 6th level Cleric and Wizard spells?
Do you get a +1 to the effective spellcasting level of your lower-level spellcasting class? I.e., a 10 Barbarian/1 Wizard can cast 2nd level Wizard spells?
Who knows.
