General officers of the line shall be appointed under the rules and regulations of the Department of Defense. Such officers shall be appointed by the Governor from among active officers of the National Guard of Alabama, who have not less than six years' active service in the National Guard of Alabama, with the advice and consent of the Senate. Vacancies in the offices caused by death, resignation, or removal while the Senate is not in session shall be filled by recess appointments by the Governor; provided, that should the vacancies in the grade of general officer of the line be in an organization in which the State of Alabama has joint interest with another state or states, the Governor is authorized to enter into a conference through the Military Department of the state with the other states concerned for the purpose of determining which state shall, with the concurrence of the other states, nominate the general officer to fill such vacancy in conformity with laws, rules, and regulations of the state and the United States written under the provisions of the National Defense Act; provided further, that staff officers allotted to this state, who are members of organizations jointly organized with other states, and who are on the staff of general or other officers who are commissioned in another state that has joint interest in a unit of the National Guard of this state, and whose appointment was approved by Alabama, shall be appointed by the Governor under the rules and regulations written under the provisions of the National Defense Act.