Using two flow graphs in the same application

GNU Radio
1

I’ve wrote an application that have two flow graphs as follows:
usrp2_source
-> fir -> file_sink. The only difference between the two are the usrp2
frequency and decimation rate.

As you can see in the annexed code, I start the first flow graph, five
seconds latter it stops and the second one starts. The problem is I’m
not
able to set the new frequency on the second flow graph.
set_center_freq
method returns None.

I tried to use the same usrp2_source for the two graphs, but the problem
persists.

Can somebody give me a hint on how make this work?

I’m using USRP2 with RFX2400 db and gnuradio rev 10780

Thanks