Mixing two signals without affecting output bias point?
As you say,  you can put the capacitors between the pot wipers and the summing resistors. You can then lose C1/C2 and R9/R10 do nothing in the above circuit.

Note that the resistance the C sees varies with the pot setting somewhat (about +/-12.5%) so the low frequency cutoff will not be completely constant, but it will vary less than it does with your present circuit (varies by ~2:1).