That's essentially what I did, what @lapasoa said, though my panning is more extreme, there is very different EQ and processing between them, compression is different, one has a little reverb but the other is dry. I'll try to be clearer. If I solo each guitar part they both, alone, play exactly correctly, they are both mono signals... the one panned hard left plays correctly out of the left monitor, the one panned hard right plays correctly out of the right monitor, and they play that way all the way through. But, when I solo them both at the same time, they still play exactly correctly... one panned hard left, one panned hard right, Until they hit that section, then, though the panning hasn't changed, and I can't detect anything different about the signal itself, the audio audibly moves to the center. Then, after that section, they both move back, audibly, to their assigned panning. I've tried phasing, and I've tried shifting one (nudging) slightly... and I'd like to say I tried other things but I don't know what else to try.
Btw, thanks for responding, I'm doing this on my own here, sometimes it can a long time to work through something I don't understand.