In this paper, we consider an overlay spectrum sharing system with two-way communications in the presence of multiple co-channel interferers under Nakagami-m fading. By employing three-phase time-division broadcast protocol, a secondary user allocates its partial power to amplify-and-forward the two-way primary signals and the remaining power to transmit its own signal. For such a set-up, we derive tight closed-form expressions of the outage probability for both primary and secondary communications. We further obtain their high signal-to-noise ratio (SNR) equivalent expressions to determine the achievable diversity gains. Moreover, we quantify the impact of co-channel interference on the spectrum sharing and present useful insights into the system performance. Numerical and simulation results conform to our theoretical findings.