To join two mono files into a stereo file, select the file you wish to use as the left channel, then on the 'Edit' tab choose 'Join', 'Join as left channel to', and choose the file you wish to use as the right channel. A new stereo file will be created. Note that you can join a mono file to itself to make a stereo file with identical left and right channels, but it's easier just to convert the file from mono stereo.
To split a stereo file into two mono tracks see Split File into its Component Channels.