The USRP B210 has 2 RF channels. When a single channel is transmitting, the receiver of the same channel is inactive. In other words, the B210 is capable of half-duplex communication on each channel. This limitation is due to the USB 3.0 interface, which uses the same data path for transmission and reception.
Assuming you have a single USRP board, a workaround would be to have 2 instances of MATLAB open. Use one instance to program channel 1 for transmission and reception for half-duplex communication, and the second instance to program the second channel as the receiver.