Herein, joint space-time multiplexing in a multi carrier multiple input multiple output (MIMO) system is considered. Previous results concerning the spatial multiplexing optimization are used to address the joint problem of carrier assignment and choice of beamforming weights in a multi user MIMO system. For practical reasons, the joint optimization is divided into two parts: subcarrier assignment based on beamformer characteristics to adjust the carrier load, followed by beamforming optimization with subcarrier reassignment. The goal is to minimize total downlink transmit power while keeping the fixed user data rates. The proposed multiplexing scheme is evaluated on a number of numerical examples.