Except as provided in this part, an employee covered by an approved voluntary plan at the commencement of a disability benefit period shall not be entitled to benefits from the Disability Fund. Benefits payable to that employee shall be the liability of the approved voluntary plan under which the employee was covered at the commencement of the disability benefit period, regardless of any subsequent disabling condition which may occur during that disability benefit period. The Director of Employment Development shall prescribe authorized regulations to allow benefits to individuals simultaneously covered by one or more approved voluntary plans and the Disability Fund.