One approach to doing that now, if you are using Pro.
Link the 2 Tracks so they pan together to the same positions.
Route one of the Tracks to a Group Channel
On the Group Channel set it to Combined Stereo Panner and put the right edge of the panner all the way to the left and the left edge all the way to the right. This will flip the panning on whatever is routed through the Group.
Now the panning on the 2 Tracks should mirror each other left & right.