Because they are?
Are you talking about 2 tracks that were recorded sperately (but the same part being played)?
Or are you talking about a track that was tracked copied and panned opposit?
Where both tracks done with the same signal path? If not, you might check the phase of your input chain, something is probably amiss.
So many things. I can pan two guitar tracks of the same part together and the sound is thicker and louder usually. I suppose if I play out of time in certain way that I could cause phase cacellation, but that would be pretty tough to keep up the whole time!
Are you sure you are even using the right phrase? Phase cancellation would result in there being a LOT less low end on the guitars when they are panned center. When they are panned hard left and right, you would get a sort of "dizzy" effect when you are in a perfect triangle with your monitors.
Are you having all these effects?
Describe better what the effect you are getting is with two guitar tracks are in the center together, and please explain how they were created (recorded).
Eddie