Any phase cancellation is because the two guitar tracks are too similar. You need to make them different.

Use two different guitars if you have them.
Or use a different strumming pattern for one of the takes.
Or detune the guitar slightly for one of the takes; some strings up, some down.
Or, after recording, shift the pitch of one take up slightly and one down slightly (10-15 cents maximum?).
Or, after recording, shift the timing of one take forward slightly and one backward slightly (10-20 milliseconds maximum? But it depends on the music, you don't want it to get too sloppy).
Or, after recording, use a plugin such as Pitch Shift, or Wow and Flutter, or a chorus, on one of the takes.
Or some combination of these.

A mono-compatible widener, such as Wider, can also be helpful as you don't have to worry about mono compatibility.