Select your audio track. Under the Modify menu, deselect the Stereo Pair option. Now, select the track you want to delete and do so. You may also need to deselect Link or at lock the video track so it won’t be deleted as well as the audio track. Select the track you want to duplicate and drag it down (or up) to the empty audio track under(above) the original while holding the Shift & Option keys. This should make a copy of your track directly under(over) the original track and in sync with it. Now, select both audio tracks again and re-enable the Stereo Pair option. Pan the tracks to center if they aren’t already there.
E=MC2+/-2db