In this paper, cross-layer design methods are proposed for jointly detecting data streams and dynamically allocating power within a multiple-input multiple-output two-phase cooperative wireless communication system operating in a spatial multiplexing configuration and with single and multiple decode-and-forward (DF) relays. The proposed joint detection and power allocation schemes employ a maximum likelihood rule, which gives rise to an efficient cooperative sphere detector and algorithms to adjust the power levels of all links in an alternating strategy. Simulation results show Bit Error Rate (BER) comparisons between the system with no cooperation, equally distributed power allocation with cooperation, and the proposed power allocation scheme for single and multiple relay systems for different conditions and scenarios.