Correct me if I'm wrong but I believe that the signals from both channels are combined before the phase inverter and then fed to the sole power amp. No stereo separation to be gained by plugging into both channels.
The only advantage I can see is if it's a wet/dry setup so you can EQ the wet and dry signal separately.
I do... Obviously, its not stereo, but you get the left and the right sounds through. This is just to get panned delays into the mix, for example. Fine for practice but we should either commit to single channel and pan it all to one side or get a second amp.
I noticed that when the two channel volumes are the same, the volume drops. I just keep the second channel a little lower volume. The channels were intended to allow two guitarists to plug into the same amp. The out of phase thing might account for the volume drop (self cancellation).