Nothing prevents two people in the same household from each holding TN status, provided each independently meets the requirements: Canadian or Mexican citizenship, a qualifying listed profession, the credential that profession accepts, and prearranged professional employment for temporary entry.
Each case stands on its own facts
Being married to a TN professional gives no advantage in a second TN case, and no disadvantage either. The second applicant's profession, credential, employer, and duties are assessed as though the first case did not exist. That also means a weakness in one file, such as duties that do not clearly match a listed profession, is not offset by the other partner's stronger position.
The listing sets out specific professions with specific credential requirements, and several of them accept defined alternatives to a baccalaureate. Place the actual duty description beside the profession's stated requirements and identify a document supporting each element. Write out what the person will do in a typical month in the United States and ask whether a reader would recognize those tasks as the listed profession's work. Crop planning, soil and tissue sampling analysis, trial design, pest and nutrient recommendations delivered to growers, and technical training of dealer staff are professional agronomic duties.