Merging Left and Right Tracks

The method I use - regularly - is to have both tracks available in Waveform view. Open the first one and go to Edit>Convert Sample Type and convert it to stereo, making sure that you have it set so that the stereo file contains all the content on the LHS. If you've got a lot to do, save this as a preset:

The resultant file will have the signal all on the left, and a flat line for the other channel. What you have to do next is mute the left channel - hit the button I've ringed to do this (the waveform will turn grey):

Click on the other file to open it, and select Copy. Go back to the first file and select Paste. That will put your other file into the unmuted RH channel. Unmute the LH channel and save the result - you're done.

I've tried all the other methods of doing this, and this is by far and away the quickest - takes a few seconds. Far less time than it's taken to explain it, in fact!