CA Rev & Tax Code Section 1622.5

In any county in which two or more boards have been created and are functioning:

(a)

The clerk of the assessment appeals boards may assign one or more members from one board to serve temporarily as members of another board, and

(b)

The board of supervisors may appoint alternate members for each board. Whenever any regular member of a board is temporarily unable to act as a member of the board, an alternate member may sit on the board and shall have the same authority to act as a regular member. Where such alternate member is likewise temporarily unable to act the clerk may assign an alternate member of the same board or of any other board to act as a member of the board and such alternate member may sit on the board and shall have the same authority to act as a regular member. In any county in which one board has been created and is functioning the board of supervisors may appoint alternate members for the board. Whenever any regular member of the board is temporarily unable to act as a member of the board, an alternate member may sit on the board and shall have the same authority to act as a regular member.