Here is an excellent primer from Motorolas Networks & Enterprise Group that addresses a lot of the issues we have been discussing here on MIMO multiantenna transmission techniques and when you can use one antenna instead of two antennas.

WiMAX Antennas Primer - A guide to MIMO and beamforming

The key takeaway is that when a CPE (customer premise equipment) has multiple antennas and is using MIMO Matrix A with Space-Time Block Coding, if an external directional high-gain wimax antenna is mounted to provide a line-of-sight (LOS) or near-line-of-sight (nlos) to a base-station, then one antenna would work just as well as two. This is because for MIMO Matrix A the datastream is replicated and separately encoded such that if one parallel path has a good enough SNR then the MIMO benefit of a second path is not as high. Same goes when using antenna diversity techniques such as MRC (Maximal ratio combining) that combines the signal from the two antennas based on their SNR's.

However if you are using MIMO Matrix B then you do need both antennas always since the datastream is divided up first then multiplexed onto the different transmit antennas (hence the name SM-MIMO - spatial multiplexing since you are multiplexing the space-domain).

Finally when the MIMO CPE radio uses closed-loop multiantenna techniques such as beamforming including EBF (Eigen Beamforming) and MRT (Maximum Ratio Trannsmission) multiple receive antennas are important to boost performance mainly for external directional antennas that can be physically seperated. The small integrated patch antennas don't provide enough flexibility to modulate the amplitude and phase array of the radiated output signal. However by sending measured channel information back to the transmitter they can affect the transmitter radiation characteristics optimally to avoid interference. A single receive external antenna is just as good as using the integrated antennas.

The Clearwire Motorola CPE i150 for example supports Maximum-
Ratio Combining (MRC), Beamforming, Open-loop MIMO Matrix A (STBC) and Matrix B (SM-MIMO).