The shared gain is a great idea. Us the other half of 1k dual pot with the op amp by connecting it from the negative input to ground via another 47 ohms resistor and a largish capacitor (470 uF at least). The feedback resistor sets the maximum gain. Note that you should really use a "C" taper pot here, though a linear "B" works well enough. You could also reverse wire a log taper "A" pot and mark it "Attenuation" but it might be confusing and odd for the users.