Cal. Rev. & Tax. Code § 1752

(a)The boards of supervisors of two or more counties may establish a multijurisdictional assessment appeals board pursuant to this chapter to equalize the valuation of taxable property within each participating county.

(b)A board of supervisors of a participating county, or any assessment appeals board created by a board of supervisors of a participating county, shall not constitute the county board of equalization during the period in which a multijurisdictional assessment appeals board exists pursuant to this chapter.