Even though it is OP, and unbalanced, its still fun to try out different combinations of spells.
This kinda works already if traits are put in the correct order (to ensure resource bars are displayed);

However, its annoying to modify save file to "un-suppress" traits, since some spells cannot be learned from physical or magical classes without resetting traits. Putting trait one by one and resetting them back.

Would be nice to have an option in the Mod Options to just allow double classes.
(Or at least to not ignore already existing traits on the character)

Dual classes already exist, so I don't know why its not supported by default (except minor issues).