A city of the metropolitan class is authorized and empowered to authorize or require a joint bridge commission created pursuant to section 14-1244 to conduct and to complete the sale of bonds provided for in sections 14-1215 to 14-1217 and 14-1223 at the same time and to the same purchaser under the best conditions obtainable, together with the bonds of the political subdivision with which such joint bridge commission is cooperating so that the benefits of a joint offering and sale may be obtained. The funds derived from the sale of the bonds of all political subdivisions cooperating may be mingled and shall be administered and expended by the joint bridge commission as one common fund. As nearly as may be, and subject to any rules and regulations which may be adopted by the joint bridge commission for that purpose, the fund shall be deposited and maintained in equitable proportions within the territory of each political subdivision, and applied to the purchase or redemption of the separate bond issues in an equitable manner. All contracts, evidences of indebtedness, and payment vouchers shall be signed by the treasurer and countersigned by each vice-chairperson.