Gain loss with dual polarization antenna?

The dual antenna/chain devices, called MIMO devices, are most effective when there is one on each end of the RF link.      Yes, the rated power is the combined power always transmitted out both antennas.  Instead of, e.g. 1 x 25dBm single antenna signal, for MIMO devices this means 2 x 22dBm signals on the cross polarizations.     However, a MIMO device  that receives the signals  will coordinate with the transmitter testing various MSC link rates in real time to determine which combination of option yields the highest thoughput of data (uniquely between each neighbor):

1) same data transmitted on both antennas ->  receiver will combine both signals essentially gaining back the 3dB, then decodes the data.
2) double the data transmitted by splitting the data stream across both antennas simultaneously ->  receiver decodes the signal from each antenna chain separately, then combines the data back to original.  

If the MIMO device transmits to a single antenna device, then it is at the disadvantage of being -3dB SNR permanently.   Well, it might get a little energy from the cross-polarization signal.

Joe