If in any county the publishers of two or more newspapers, at least one of which by reason of its location and circulation is entitled to be selected as a county official newspaper, have entered into an agreement to publish the official proceedings or have united in a request to have their publication selected for such purposes, and such agreement or request has been filed with the board of supervisors prior to the naming of the official newspapers, the board of supervisors shall designate each of them a county official newspaper, but the combined compensation of the newspapers so requesting or agreeing, added to that of the other official newspaper or newspapers, if any, shall not exceed the combined compensation allowed by law to two official newspapers in counties having a population below fifteen thousand or to three official newspapers in counties having a population of fifteen thousand or more.