Are you mixing in a DAW?
If so, just zoom in pretty close to both the tracks and "slip edit" one of them so that the waveforms are both going + and - at the same time.
I have had double tracked stuff come to me that was somewhat out of phase between a DI and mic signal. They don't even have to be 180 degrees out of phase for this to be a problem. Anyway, you just move one of the track in the time line to match the other and you have pretty much fixed the problem. It will be obvious when you zoom in fairly close.