Cognitive full-duplex relaying systems offer the advantage not only of increasing spectral efficiency by spectrum sharing but also of extending the coverage through the use of relays. In this paper, a novel two-phase spectrum sharing protocol for two-way full-duplex decode-and -forward (DF) relaying, is proposed. Analytical expressions for the outage probabilities of primary and secondary users are derived and the theoretical results are approved via computer simulations. Obtained results are compared with the outage performances of two-phase full duplex amplify -and -forward (AF) relaying and three-phase halfduplex DF relaying schemes. It is observed that the outage performance of primary user increases when the system residual loop interference is small enough.