I believe that if you set gr-osmosdr blocks to num channels == 2, you will get two streams and two sets of channel configuration parameters (gain, antenna, filter bw, freq). So the only way to access the second channel (channel 1) is to use the second stream and second set of parameters on this block.
Now it is a bit of a software limitation in gr-osmosdr, because I bet you would be looking to have a single stream and parameter set, just on channel 1 instead. gr-uhd blocks actually have a second channel list argument that performs mapping from the parameters to a physical channel. So this uhd source block for example would have 1 channel, with the channels list selected to be “[1]”.