I was told somewhere that a way to increase spell save DC was by multiclassing and taking a spell in each class multiclassed.
If this is the case, a Sorcerer and Warlock multiclass would be able to add Charisma modifier twice to spell save DC if both have the spell. I'm unsure if it was the ability score modifier or the proficiency that was doubled – are either correct?
Best Answer
No, they do not stack
Per the basic rules on multiclassing, specifically under spellcasting:
Spells you learn through a class are associated with that class, even if they are learnable/castable by other classes. If I learned Hold Monster as a Warlock for example, I would have to cast it as a Warlock spell, even if I multiclassed into Sorcerer or Wizard.
Learning through another class does allow you to choose which to use, but you still only cast from a single class, so you wouldn't be able to add your proficiency/ability modifier twice through multiclassing.