If the area to be regulated and restricted is situated in two or more counties, a joint planning commission may be established. Membership of such a joint planning commission shall consist of five members from each county planning commission to be appointed by the chairman of the respective county planning commissions. Each joint commission shall make a preliminary report and hold public hearings thereon as is provided in the case of county planning commissions before submitting its final report and recommendations to the respective county planning commissions of each county concerned.