When ten percent of the voters of each of two adjoining counties, file a petition with the Department requesting that such counties be joined for the purposes of this Act, the Department shall immediately notify the county clerks of such counties that such a petition has been filed, and if neither county board has elected a county airport commission as in this Act provided, no further action shall be taken by either county board in the creation of a Commission, until notified by the Department that such petition is insufficient or that the joining of the two counties will not be in the public interest. The signatures of the voters of one county shall not be intermingled with the signatures of the voters of the county to which it is to be joined.