A city or town may by ordinance provide that its water system, sewerage system, and garbage and refuse collection and disposal system may be acquired, constructed, maintained and operated jointly, either by combining any two of such systems or all three. All powers granted to cities and towns to acquire, construct, maintain and operate such systems may be exercised in the joint acquisition, construction, maintenance and operation of such combined systems: PROVIDED, That if a general indebtedness is to be incurred to pay a part or all of the cost of construction, maintenance, or operation of such a combined system, no such indebtedness shall be incurred without such indebtedness first being authorized by a vote of the people at a special or general election conducted in the manner prescribed by law: PROVIDED FURTHER, That nothing in chapter 51, Laws of 1969 ex. sess. shall be construed to supersede charter provisions to the contrary.