As a character progresses from one [level] milestone to another, they must spend time testing their progressing theories, knowledge and understanding. This usually is represented as quiet and uninterrupted time, in a place where they can contemplate and test themselves.
Multiclassing must be reflected in time and resources required to learn the new skills and traits and integrate them seamlessly into their existing classes capabilities. Multiclassing is the simple equivalent of splitting aptitude ratings – it provides more flexibility, but less direct power.
When using d20 Aptitude Approach: Multiclassing Ability Score Minimums increase for each Class added. ex: A second Class will have a 14 for a minimum, a third class would have a 15.
For entities that have Inherent Specialization, it is harder to multiclass.
Or equivalent for longer lived species…