Depending on what a song needs, as opposed to automatically double tracking the same parts - which may indeed be what a song needs - with guitars in general, I always consider having the parts play off of each other, and leaving room for each part, as well as room for other instrumentation.
In my current song project, I have acoustic guitars playing 'simplified' rhythms. In one section, the left channel playing mostly simple slow one strums, while the right channel is playing up & down strums. In other sections, I may do flat picking, and the left & right guitar tracks work off one another.
In the same song, I have electric guitars that are double tracked. At some sections doing the same things, which inconsistencies can be favorable, as well as they reinforce each other. At another section I'm playing counter rhythms off of each other in the left & right field. One has to really be careful not to make a big mess of things...with the guitar parts themselves, and also to not hog the entire arrangement up. Simple creative experimentation and careful considerations is needed during the process.
Even slight variations of the same parts can really make a difference, but creatively working simple parts off of each other really can open things up, while not necessarily dominating an arrangement.
