Cooperation diversity schemes employing space-time block coding (STBC) techniques have been proposed for wireless networks to increase systems capacity and coverage, by allowing multiple relay nodes distributed in space to assist the transmission between a given pair of terminals. Conventional relaying strategies, such as amplify-and-forward (A&F) and decode-and-forward (D&F), are designed by considering unidirectional communications via relays, which are referred to as one-way relay channels. Recently, it has been shown that, when there is consistent traffic in both directions, the use of two-way relaying protocols, which explicitly account for the bidirectional nature of the data flow, can potentially improve the network performance. In this paper, capitalizing on randomized STBC, a coding rule which has been originally developed for D&F one-way relay channels, fully decentralized cooperative communication schemes are proposed for both A&F and D&F relays, where each relay is unaware of both the effective STBC being employed by the other nodes and the number of cooperating stations. Numerical results are provided to highlight the effectiveness of the proposed two-way relaying schemes in comparison to their one-way counterparts.