This says that two channels on the one block should work.
What are the parameters that you are setting on the second channel. The two channels are linked and have restrictions, so there are probably very few parameters that can be changed.
Reading through the source code for gr-osmosdr (gr-osmosdr/lib/soapy) it just hands everything back to Soapy, so it would be whatever the SoapySDR bindings for LimeSuite devices using the LMS7 API says that the device can handle (LimeSuite/SoapyLMS7), And speed-reading through Settings.cpp, SoapyLMS7.h and Streaming.cpp it looks to me if you specify bad parameters you will get no feedback on the B channel (with regard to frequency anyhow).
EDIT: Sorry was speed reading a bit too fast, that is only when applying corrections. But the A and B channels are linked, you are probably going to have to dig through the datasheet, examining it in great detail and with great care, to find which parameters can be tweaked and by how much. Reading through the code everything is passed back to the LimeSDR API.
EDIT TWO: After reading through LimeSuite/SoapyLMS7/Settings.cpp and LimeSuite/src/lms7002/LMS7002M.cpp I am guessing that if you are trying to use two different frequencies then you need to set both to the exact same frequency and adjust the NCO Rx/Tx for channel A and NCO Rx/Tx for channel B to offset from that centre frequency. I would start by looking at the RXTSG_NCO/TXTSG_NCO parameter for each channel.
EDIT THREE: There does not appear to be any option in the osmocom source/sink block in gnuradio companion to pass the RXTSG_NCO/TXTSG_NCO parameters per channel. So if you want access you will need to be editing the python code by hand and ignore the gui.