Yep, a 2nd TL, one op-amp for each output, if you aim at a mixer that does not invert the phases, otherwise you can leave those out. The +inputs to GND make that the opamp makes a virtual GND at its - input because an opamp with a feedback loop (resistor connecting output and - input) will try to reduce the voltage difference between its -input and +input to zero. If it succeeds, and it will, the -input will get the same voltage as the +input, thus the -input will become 0 volt, so is virtually connected to GND. In that condition you can calculate the resulting output voltage as the ratio of the input the feedback resistor and the resistor connected to the input (times -1) times the input voltage. More details here.
The capacitors in their original position decouple the inputs from the opamp, but not the inputs from one another because they are connected to one another via the potentiometers and resistors at the input stages of the mixer. So a DC value on input one would also in part be connected to the other inputs (and the outputs they get their voltages from). You may ask yourself whether that is what you want.