Panning between two active audio signals
You are going to have to experiment to find the best panning curve for your application (and preference).  Start by changing R3 to the 50k pot that you want to use.  Increase the gain of the circuit by increasing the value of the feedback resistor (the extreme right-hand resistor) from 10k to 47k.  Adjust that value to suit your prreferences.
If you leave the input resistors (the two left-hand resistors) at 10k, the panning pot will not have as much effect at the center range of travel as at the ends.  If you want the pan action to be more smooth across the range of travel, increase the value of the input resistors and increase the value of the gain-set resistor to compensate for the added attenuation.
Be sure to use a decent-quality low-noise op-amp.  A LM358 is far too noisy for this application.
[Edit]
Andy brings up a very valid point: the wiper of the pot can't go to ground unless you are using a bi-polar supply for the op-amp.  Either connect the pot wiper to the reference supply (and add a large-value bypass capacitor to ground) OR just put a large value capacitor in series with the wiper to ground.