USRP B210 for 2x2 MIMO support

I'm trying to transmit data from two TX ports on a B210 USRP to another B210 USRP device for reception. However, the TX/RX B port doesn't seem to be working. On the receiving device, I only receive the signal from the TX/RX A port. I've tried transmitting signals on each port, but I only receive the signal from the TX/RX A port. Please help me!

Below is my program.

txRadio = comm.SDRuTransmitter( ...
    'Platform',            'B210',         ...
    'SerialNum',           serialNum,      ...
    'ChannelMapping',      [1 2],          ...
    'CenterFrequency',     2450e6,    ...
    'Gain',                60,        ...
    'MasterClockRate',     30.72e6,   ...
    'InterpolationFactor', 8);

comm.SDRuTransmitter requires one of the following:
Wireless Testbench Support Package for NI USRP Radios
Communications Toolbox Support Package for USRP Radio