In this paper we propose a two-way spectrum sharing protocol for the overlay cognitive radio network consisting of two primary users (PUs) and two secondary users (SUs). One of the SUs acts as a relay to assist the communication between the PUs while at the same time allowing for two-way transmission with the other SU. The communication process is done in two phases. In the first phase, the PUs and the other SU are allowed to transmit simultaneously. The relay can use successive decoding to decode the message of the SU and then the PUs messages. In the second phase, the relay can use superposition coding to generate a new message composed of the XORed messages of the PUs and a new message for the SU. The performance of the protocol has been analyzed by deriving the outage probabilities of the PUs and SUs. Analytical and simulation results show that the proposed protocol offers efficient spectrum utilization and improved system capacity over the three-phase and two-phase spectrum sharing protocols with one-way SUs transmission in the literature.