Forum: GNU Radio how to set two daughter board(RFX2400) by USRP1

Hey everyone ,
im using two RFX2400 and trying to workout benchmark_tx.py

i don't know how to set both daughter board working ,

here is the help information :
"-T", "--tx-subdev-spec", type="subdev", default=None,
                          help="select USRP Tx side A or B")

Can some1 tell me how to set this ?
thanks alot for your time