The best way to do this is pan both of the mono tracks back to the center. If you have the recording named as both tracks, you should give one track for instance, Track 1 - song name1 second track - song name2. After you name them, select both track waveforms, then bounce the two tracks. Then in the browser, head over to your file folder, find the song name, click the drop down arrow, look for bounces. Open the bounce file. You should see the two tracks that you have bounced. Select those two tracks in the bounce file, right click, look for merge to stereo file. Select the merge to stereo file. Go back to your bounce folder. You should see a new stereo file that was created. In other words a waveform that was not there before you merged to stereo. Go back to the song page, delete or cut both mono tracks. Drag the newly created waveform to your stereo track and bam! Done!