If any other school district or districts in the Town of Bennington or in the adjoining Town of Shaftsbury shall at any time, by their vote at a meeting duly warned for that purpose, signify their desire to become united with the Graded School District, and if the Graded School District at a meeting duly warned for that purpose shall vote to agree to the union, the additional District shall to all intents and purposes become and form a part of the North Bennington Graded School District, and the union of the districts may be made upon the terms in regard to the school property of each as may be mutually agreed upon by the districts.