Receiving from Rx-A and Rx-B of a single daughterboard

Hello, all.

We are developing a program that uses Rx-A and Rx-B. Rx-A is used to
sense
the spectrum and, after that, Rx-B to receive FM signals. Is it only
possible to use A and B simultaneously in a program using one USRP?
First, the program call Rx-A. After some time, Rx-B is called and the
error
below is given:

usrp_open_interface:usb_claim_interface: failed interface 2
could not claim interface 2: busy
usrp_basic_rx: can’t open rx interface

Traceback (most recent call last):
File “/home/ed02/Documentos/usrp_wfm_rcv.py”, line 304, in
app = stdgui2.stdapp (wfm_rx_block, “USRP WFM RX”)
File “/usr/lib/python2.6/dist-packages/gnuradio/wxgui/stdgui2.py”,
line
36, in init
wx.App.init (self, redirect=False)
File
“/usr/lib/python2.6/dist-packages/wx-2.8-gtk2-unicode/wx/_core.py”,
line 7935, in init
self._BootstrapApp()
File
“/usr/lib/python2.6/dist-packages/wx-2.8-gtk2-unicode/wx/_core.py”,
line 7509, in _BootstrapApp
return core.PyApp__BootstrapApp(*args, **kwargs)
File “/usr/lib/python2.6/dist-packages/gnuradio/wxgui/stdgui2.py”,
line
39, in OnInit
frame = stdframe (self.top_block_maker, self.title, self._nstatus)
File “/usr/lib/python2.6/dist-packages/gnuradio/wxgui/stdgui2.py”,
line
60, in init
self.panel = stdpanel (self, self, top_block_maker)
File “/usr/lib/python2.6/dist-packages/gnuradio/wxgui/stdgui2.py”,
line
81, in init
self.top_block = top_block_maker (frame, self, vbox, sys.argv)
File “/home/ed02/Documentos/usrp_wfm_rcv.py”, line 82, in init
self.u = usrp.source_c(which=0) # usrp is data
source
File “/usr/lib/python2.6/dist-packages/gnuradio/usrp/usrp_swig.py”,
line
1651, in source_c
return _usrp_swig.source_c(*args, **kwargs)

RuntimeError: can’t open usrp

The code is based on the programs spectrum_sense.py and
usrp_wfm_rcv2.py.
How can I use Rx’s A and B in a program, but not in the same time (and
calling usrp_source only once)?

Thanks for your attention,