Interesting. I have a couple of questions.
If you MC two (or more) classes that use the same casting stat, do you add the ability mod just once, or once for each class? For example, if the character were Sorcerer(3)/Bard(3), with CHA 16, would it be (3*4 + 3) + (3*4 + 3) = 30, or 3*4 + 3*4 +...