I keep them panned 100% left for one guitar and 100% right for the other guitar.
A good example of that:
[VIDEO]http://www.youtube.com/watch?v=lL0ucHt-Z9k]‪Exsanguination - Convergence (HD)‬‏ - YouTube[/VIDEO]
3:49 - 3:57 : Unison riff
3:57 - 4:07 : Harmonized riff
As just one example.
If you're only listening to the music through one speaker, you're obviously not going to get the harmony... but if you're only listening through one speaker, you have more serious problems than that to deal with.