RXF2400 as receiver by using benchmark_rx.py, which will receiver the signal(RX2 or TX/RX, or both)

Hi all,
I’m new in gnuradio and USRP, recently, when i run benchmark_rx.py, i
found a problem which confused me a lot:
I use one USRP(equiped with a daughterboard of RFX2400, running
benchmark_tx.py) as transmiter and another USRP (RFX2400 runing
benchmark_rx.py) as receiver, as i only have one antenna, so i connect
it to RX2 and run benchmark_rx.py at the receiver, as i expected, the
result is nearly 100% “ok”, but when i disconnect from RX2, it results
some “false”, that is narmal.But again, i connect the antenna to
TX/RX(not RX2), the result comes back to 100% “ok”, i don’t understand,
could it be that when RFX2400 is used as a receiver, both TX/RX and RX2
receive the signal, if not ,how do they work?
I learned that if i want to select the rx antenna, i could use
select_rx_antenna(‘RX2’), but even i did this ,it does nothing
i think MUX may be something related, but i’m not sure.
Any help will be appreciated.