The board shall establish the boundaries of the seven divisions from which directors are elected in accordance with Chapter 6 (commencing with Section 71540) of Part 4. The additional directors shall be elected at a special district election which shall be called by the Board of Directors of Three Valleys Municipal Water District on the next available election date established by Chapter 1 (commencing with Section 1000) of Division 1 of the Elections Code that is 90 or more days after January 1, 1989. The two additional directors so elected and qualified shall take office on July 1, 1989, and shall hold office until their successors, who shall be elected at the district general election in 1992, take office. Thereafter, the two additional directors shall be elected to terms of four years.