UHD: USRP source issue when using 2 channels

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Am I the only one facing an issue with fetching data from 2 channels of a B210 
receiver ?
When addressing a single channels (USRP Source -> file sink) all goes well, but
whenever I activate a second channel (Num Mboards=1, Num Channels=2) I get an 
error message

tb = top_block_cls()
File "/tmp/GPS.py", line 103, in __init__
self.connect((self.uhd_usrp_source_1, 1), (self.blocks_null_sink_0, 0))
File ".../gnuradio3.8/lib/python3.7/dist-packages/gnuradio/gr/hier_block2.py", 
line 48, in wrapped
func(self, src, src_port, dst, dst_port)
File ".../gnuradio3.8/lib/python3.7/dist-packages/gnuradio/gr/hier_block2.py", 
line 111, in connect
self.primitive_connect(*args)
File ".../gnuradio3.8/lib/python3.7/dist-packages/gnuradio/gr/runtime_swig.py", 
line 5461, in
primitive_connect
return _runtime_swig.top_block_sptr_primitive_connect(self, *args)
RuntimeError: port number 1 exceeds max of 0

whatever sink (file, null) is attached to the UHD output. I am facing the issue 
with custom 3.8.0.0-rc2 (Python 3.7.5) or the current Debian/sid 3.8.0.0 
(Python 3.7.5) versions of GNU Radio Companion.

The flowchart with the issue is at http://jmfriedt.org/GPS.pdf

Thanks, JM