As the number of wireless devices keeps increasing with users demanding diverse applications requiring higher data rates, making cellular networks energy efficient has now become extremely crucial. Network infrastructure sharing among mobile network operators (MNOs) is being considered as a viable and efficient option for energy cost reduction. In this article, we propose a novel technique for traffic sharing between two MNOs for improving their energy efficiency (EE). For the proposed BS sharing, we introduce the theory of coopetition for real-time interaction between the two MNOs. Unlike the full-cooperation and full-competition schemes, proposed coopetition mechanism implements both cooperation and competition between the two MNOs. Through this coopetition, users of one MNO is re-associated with the base stations (BSs) of another MNO keeping the provision of switching BSs into sleep mode having no active users. Performance of the proposed scheme is evaluated through extensive simulations exhibiting a substantial gain in EE. Furthermore, EE performance of the proposed scheme is compared with that of full-cooperation, full-competition and no-interaction schemes acting alone.