The governing boards of two or more school districts, in none of which the employees are entitled to the benefits of a pension or retirement system maintained by a city, city and county, or county in which the district, or a part thereof, is located, meeting in joint session, may in their discretion, submit to the qualified electors of the districts, and upon petition signed by not less than 10 percent of the qualified electors of each district the governing boards shall submit to the qualified electors of the districts the proposition of establishing a joint plan for a retirement salary to be paid by the districts to all teachers fulfilling the requirements of the plan in the public schools of the districts and to other employees of the districts as the governing boards may determine, or, in the case of a petition, other employees specified in the petition, and to provide, out of funds of the districts, a portion of the cost thereunder, including expenses incident to the administration of the plan.