I am going to start with the question that is easiest to answer: yes, it is possible to include a clause to that effect.
With respect to double incontinence, that is a reversible condition. For example, the person might have a urinary infection or a reversible gastric or intestinal problem. We cannot determine a stage based on a single symptom that may be temporary. Instead, there would have to be a set of symptoms that were considered to be irreversible, in order to say that a person has reached a threshold that justifies medical assistance in dying.