California Code, Government Code - GOV § 56123

Except as otherwise provided in Section 56124, if a proposed change of organization, reorganization, or determination of a sphere of influence applies to two or more affected counties, for the purpose of this division, exclusive jurisdiction shall be vested in the commission of the principal county.  Any notices, proceedings, orders, or any other acts authorized or required to be given, taken, or made by the commission, board of supervisors, clerk of a county, or any other county official, shall be given, taken, or made by the persons holding those offices in the principal county.  The commission of the principal county shall provide notice to the legislative body and the executive officer of all affected agencies of any proceedings, actions, or reports on the proposed change of organization or reorganization.  Any officer of a county other than the principal county shall cooperate with the commission of the principal county and shall furnish the commission of the principal county with any certificates, records, or certified copies of records as may be necessary to enable the commission of the principal county to comply with this division.