California Code, Public Resources Code - PRC § 5505

If the petition contains names of electors residing in a county other than the one having the largest area within the proposed district, and does not contain the requisite number of signatures of electors residing in the county having the largest area within the proposed district, a duplicate original of the petition shall be filed with the board of supervisors of the other county, and the clerk of the board of supervisors of the other county shall likewise check and verify the signatures to the petition and certify the result to the board of supervisors of his county, and thereupon copies of the clerk's certificates shall be transmitted to each county affected.