Two-way relaying is an effective approach to exchange information between two simultaneously transmitting sources in wireless fading environment. Space-time non-regenerative relaying aids to keep the relay hardware simple and cheap. Accurate estimates for the channel and carrier frequency offset (CFO) are required by the receiver to reap the benefits from relaying. In this paper, we propose a training frame format for relay transmission and formulate a subspace based CFO and channel estimator. The estimation problem is shown to be similar to the joint CFO and channel estimation in a multiuser uplink system. Furthermore, the relay and source nodes are not required to adjust their carrier frequencies. Numerical results show that the mean square error performance of the proposed estimators can reach their corresponding Cramer-Rao lower bounds.