[where02190]

I'm guessing you either
A: run through some sort of stereo processor to achieve the stereo guitar or
B: double mic

In neither case, since the guitar dissapears in mono, one track is 180 degrees out of phase with the other, essentially cancelling themselves out when summed to mono. Look at both tracks, and invert (using the invert plugin or a phase invert on a plugin like a digi eq you may already ahve on the track) the one that is out of phase (starting from zero the waveform goes down then up). You no whave tow trakc s that are phase coherant, and will trnaslate to mono perfectly.