Upon its creation, the commission’s governing board must consist of two members from each county appointed by the governing body of that county, plus one additional member to be appointed from the county first enacting the joint ordinance. Appointment of that additional member must rotate among the counties after each term in a sequence corresponding to the order in which the counties enacted the ordinances. Members serve for terms of four years and until their successors are appointed and qualify, except that for the commission’s initial appointments, each county’s governing body must designate one of its appointees to serve a two-year term after which the successor shall serve for a term of four years. Regardless of the date of appointment, terms end on July first of the applicable year. Vacancies must be filled in the manner of the original appointment for the unexpired portion of the term.