When a defendant enters a combined plea of "not guilty and not guilty by reason of insanity," the court may appoint a sanity commission as provided in Article 644 to make an examination as to the defendant's mental condition at the time of the offense. The court may also order the commission to make an examination as to the defendant's present mental capacity to proceed. Mental examinations and reports under this article shall be conducted and filed in conformity with Articles 644 through 646.