The respective boards of county commissioners of two or more counties may by agreement and resolutions of the respective boards of county commissioners employ a joint county director of tax equalization who shall act as county director of tax equalization for each of the counties participating in the agreement. The salary and expense of the joint county director of tax equalization and that of the office and staff must be prorated among the counties participating in accordance with the assessed valuation of the counties concerned or upon any other basis as may be agreed upon by the respective boards of county commissioners. The respective boards of county commissioners, acting jointly, shall appoint the joint county director of tax equalization on the same basis and in the same manner as a county director of tax equalization may be appointed for a single county. The joint county director of tax equalization may be discharged upon the resolution of the board of county commissioners of any county participating in the agreement. Any participating county may withdraw from the joint agreement upon resolution of the board of county commissioners and by giving written notice to the boards of county commissioners of the other participating counties at least ninety days in advance of July first of the year of withdrawal. The joint county director of tax equalization shall have all the powers and duties of the county director of tax equalization of a single county and shall keep all records of assessment for each county entirely separate from the records of other counties served by the joint county director of tax equalization. The governing boards of a county and of any city, or any township, may by agreement and resolutions of the respective boards employ a joint county director of tax equalization and city or township assessor.