What you've drawn is more like a stereo width control.
If you think of a stereo feed as a mono signal fed to both channels, plus a 'difference' signal, added to the left and subtracted from the right, then: the mono signal will pass from in to out unamplified (whatever the gain setting). The difference signal is amplified (between 1x and 'lots') then added to the left/subtracted from the right.
So, not a two-channel gain control I'm afraid.