Creating a stereo track from two mono tracks

A little background:  I have an acoustic piano with a digital player mechanism.  A proprietary computer controls the the solenoids that activate the keys.  That computer uses data encoded into an audio stream (which kind of sounds like a 90's modem if you were to listen to it.)  Some of the playback media, in addition to activating the keys on the piano, also have a channel of audio for playing back singing or orchestrations synchronized with the piano playback.  Those files are formatted so the left channel is the orchestra/singing and the right channel is the data for the piano playback.  

I have one stereo audio file with singing and one stereo audio file with the left channel empty and the right channel with the data signal audio.  

I need to a) convert that first stereo file with music and singing to mono and b) import either the left or right information in that mono file into the unoccupied left channel of the second file with the data signal audio.  

I feel that this must be super easy, but I'm drowning in features.  I've done a lot of work in Logic Pro with MIDI, but I'm inexperienced when working with audio.

Thank you in advance for any guidance.

There are many ways to skin this cat.

You can try this:

1. Insert a Gain plug-in on the singing track.
   
2. Turn on Mono.
   
3. Pan the track hard left.
   
4. Bounce your project including the data track.

When it comes to editing audio like that, I always recommend an application I've been using for years. I love it. it's called Sound Studio. It costs $30 and it's really good. Besides 2 small details that I already suggested they could change/add, it's the perfect audio editor for me (https://felttip.com/ss/).

With Sound Studio you can open an audio file, choose Mix To Mono and there you go. If you have a mono file and want to create a stereo file, you create a new empty stereo file, go to the mono audio, copy and then you can paste only on the left or right or both. 

If you don't want to buy this app, then I think Eric's suggestion will do the trick! 

Hope it helps