Any 2 or more counties may, with Department approval, appoint or elect the same person as county supervisor of assessments for each of such counties and may by agreement provide for the appropriate share of the salary and expenses of the official. In any case where a supervisor of assessments is shared, the aggregate population of the 2 or more counties shall be considered as the population when determining the compensation of the official under Section 3-40. If a county board desires to appoint as county supervisor of assessments of that county a person who is the appointed or elected supervisor of assessments of another county, the person shall not be required to take the examination given by the Department and shall not be required to accept the appointment.