How does the Law of Agency make it possible for a patient to sue both the medical assistant and the phyiscian for negligence committed by the medical assisant?

Because the medical assistant is the legal responsibility of the physician and are acting under their direction.

Medical assistants often have training at unaccredited for-profit institutions, or in many cases in states that do not require it, they have no training at all but are simply trained by the doctor while working at the job. This makes them the direct responsibility of the doctor supervising them. It also makes them dangerous.

This is why many doctors refuse to hire medical assistants and only hire nurses with real college degrees.