Welll do exactly what you stated in your first contribution.
Your Mono 'soundtracks' probably get loaded with two channels (i.e. the data matrix contains 2 columns), one representing the left and one representing the right stereo channel. But both would contain the same information. Thus when playing it, you get the same sound out of each channel, giving you the impression that the sound source is directly infront of you.
Now if you replace one of the columns of the two with one of the columns from the other soundtrack, then when you play the composite soundtrack, you will get one set of contents in the right ear, and the other in the left ear.
The only thing to be careful of, is that the wav files were sampled at the same rate, and you truncate the length of the longest soundtrack to exactly equal to the length of the shorter one, so that combining the two becomes relatively trivial.
I guess you already know how to read in the original wav file,and to play the composite waveform through your sound system. If not come back.
Your original contribution seemed to suggest that you already had an excellent idea how to proceed, as you hadn't actually asked a question.