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This paper considers the relay subset selection problem in a two-way amplify-and-forward relay network, where each user is equipped with multiple antennas and each relay is equipped with a single antenna. In a two-way channel, two users exchange data with each other using a relay subset. With the optimal relay subset selection scheme, the system can reduce the hardware burden while preserving the diversity benefit. However, the optimal relay subset selection algorithm requires an exhaustive search of all possible combinations to determine the optimum subset in order to achieve the maximum sum-rate. Thus, this results in high computational complexity. In order to reduce the computational load while still maximizing the achievable rate, a cross-entropy (CE) method is introduced to search for the optimal relay subset. Simulation results indicate that the proposed CE method can guarantee a result that is within 98% of the optimum sum-rate obtained by the exhaustive search method with low computational complexity.