In furtherance of the policy of this state as set forth in section 70-1501, suppliers of electric power and energy, including public power districts, nonprofit corporations, public power and irrigation districts, individual municipalities, registered groups of municipalities, public corporations, electric membership associations, cooperatives, and any other entities, are authorized to enter into written agreements between or among themselves which (1) prohibit, limit, or set conditions on the right of any party to the agreement to sell power and energy at wholesale to any entity which is then or thereafter served by another party to the agreement or to any entity listed in the agreement as a customer of another party to the agreement or (2) require any party to the agreement which sells power and energy at wholesale to any entity which is then or thereafter served by another party to the agreement or to any entity listed in the agreement as a customer of another party to the agreement to purchase power and energy from another party to the agreement.