We consider in this paper distributed beamforming for two-way cognitive radio networks in an effort to improve the spectrum efficiency and enhance the performance of the cognitive (secondary) system. In particular, we consider a spectrum sharing system where a set of decode-and-forward (DF) relays are employed to help a pair of secondary transceivers in the presence of multiple licensed (primary) users. Among the available relays, only those that receive the signals reliably participate in the beamforming process, where the optimal beamformer weights are obtained via a linear optimization method. We derive closed-form expression for the probability distribution function (PDF) of the total end-to-end signal-to-noise ratio (SNR) at the secondary transceiver. We also derive closed-form expressions for the outage and error probabilities over independent and identically distributed (i.i.d.) Rayleigh fading channels. Numerical results show the effect of beamforming in enhancing the secondary system performance in addition to mitigating the interference to the primary users.