How to have two attributes, together, be unique?

Solution
class User(db.Model, UserMixin):
     __table_args__ = (db.UniqueConstraint('first_name', 'last_name'),)
    id = db.Column(db.Integer, primary_key=True)
    first_name = db.Column(db.String(15), nullable=False)
    last_name = db.Column(db.String(15), nullable=False)
    email = db.Column(db.String(120), unique=True, nullable=False)
    password = db.Column(db.String(60), nullable=False)

    def __repr__(self):
        return f"User('{self.first_name}', '{self.last_name}', '{self.email}')"