I generally just use the same guitar and amp settings for both sides. I have a symmetry fetish and I don't want a mix that has more of one frequency on one side than the other.
If I'm doing more than one double, I will change the amp/guitar settings and do two more passes. Making sure that the first and second layers complement each other.
I would advise against a symmetry fetish, or a "fetish" of any kind when it comes to how you use your planes of space in a mix. There are five planes of a mix contained within four dimensions (time being the fourth). They are as follows: Frequency (up to down), panning (left to right), balance (front to back), reflectivity (far to near), and contrast (sparse to dense).
Now contrast, while not always necessary or called for, certainly is a valuable tool in mixing. When you keep all of your mixes symmetrical at all times, you're eradicating an element of effective contrast from your mix. Using some asymmetry in your mixes can have the effect of making your symmetrical sections more powerful, and this is exceptionally useful as a dynamic. I use this tool all the time when mixing, because effective pop and rock mixes tend to require less dynamic range than we'd like to use as mixers, mostly because people tend to listen to music in environments that make a large dynamic range problematic. If you have parts on both sides at all times of your mix, you're also going to diminish the illusion of a change in density. Food for thought.
Doing anything as a matter of course based on your personal preferences is inadvisable when mixing (and producing), as it puts your sound above and beyond that of the artist. There are times when a dbl tracked guitar, using the same amp, same guitar, and same player are the desired effect, regardless of my overall preferences. And while as a producer I TEND to shy away from that sound, it would be nothing short of foolish for me to completely remove that particular tool from my arsenal. I would suggest that the same goes for you where symmetry is concerned.
Contrast is an excellent tool in mixing for assisting with forward push through the song. A great songwriter will use techniques that push the listener forward through the song. A great mix will often assist with forward push through the use of smart arrangement techniques.
Enjoy,
Mixerman