as I said: you can use a higher CQI, since you have an SINR improvement due to diversity. Using your 2×2 antennas for spatial multiplexing is only the right choice for very good channel conditions. Therefore it is only chosen in a few cases.
In essence: Even when not transmitting over 2 streams, you improve your througput through higher SINR and therefore higher CQI.
You can also check the improvement in throughput and average CQI when using a 2×1 or 1×2 antenna setup. The RI is constant one, but the SINR is improved.