The physical-layer network coding (PNC) approach provides improved performance in many scenarios over "traditional "relaying techniques or network coding. This work addresses the generalization of PNC to wireless scenarios where network nodes have multiple antennas. We use a recent matrix decomposition, which allows, by linear pre- and post-processing, to simultaneously transform both channel matrices to triangular forms, where the diagonal entries, corresponding to both channels, are equal. This decomposition, in conjunction with precoding, allows to convert any two-input multiple-access channel (MAC) into parallel MACs, over which single-antenna PNC may be used. The technique is demonstrated using the two-way relay channel with multiple antennas. For this case it is shown that, in the high signal-to-noise regime, the scheme approaches the cut-set bound, thus establishing the asymptotic network capacity.