If you want to do it with just resistors, you could make R16 and R17 somewhere around 4k7 each, and add a few uF of bypass capacitance around one or both resistors. 1k would give a stiffer reference if you don't mind the additional power consumption.
If you can spare an op-amp section, you can get a better (stiffer) ground reference than you'll get with just resistors, with lower power dissipation (I think). Use R16 and R17, and a small cap from the junction point, to create a Vcc/2 reference, but do not "ground" this directly to your internal reference point (the rectangular-looking ground symbol). Instead, feed this to the noninverting input of an op-amp section, feed the op-amp output back to the inverting input (i.e. create a unity-gain follower), and use the op amp's output as your ground reference. In this arrangement R16 and R17 can be high-value (100k?) as their junction point will be looking into a high-impedance op amp input.
Pretty much... each of the op amps' noninverting inputs, and the "bottom ends" of the potentiometers, as you have drawn them. *NOT* the V- input to the op amp(s), of course.
You might want to add "pop preventer" resistors at the inputs and outputs... say, 100k to DC ground, from the "outside" end of each of the DC-blocking capacitors.