Full Duplex with two USRPs and four FLEX900 daughterboards

Hi,I am new to USRP. Could anyone help me with this problem? Now I try
to use two USRPs and four FLEX900 daughterboards with one using Tx/Rx
and another Rx2 port for transmitter and receiver. Is it possible to
achieve full duplex at same frequency(915MHZ)?I have measured the
isolation between Tx/Rx and Rx2 ports on one daughterboard,which is
about 15-20dB. So I need to use two daughterboards. Then isolation
between Tx/Rx port on one daughterboard and Rx2 port on another
daughterboard is 60-65dB. But I still cannot achieve full-duplex even if
I do set_auto_tr(True) and usrp.selected_subdev. One receiver will only
receive data when its transmitter on the same daughterboard finish
transmitting. Thanks.