Double tracking can be a great way to add width, depth, and character to your guitar tracks. Doubling is where you play two different takes of the same part and play them back simulataneously. My favorite way to double is to take two audio tracks, group them together and pan one hard left and one hard right. Then I record one track of guitar and then try to double the part as close as possible on the other track. This creates a nice, natural chorus effect as well as adding width and movement to the part. The track will seem to pan left and right subtly, as the natural differences in volume play between the two tracks.
Doubling guitars and panning them hard left and right also gives other advantages when mixing. For one it adds width while also allowing your guitar sound to be more present in the mix. Also, having the guitars panned hard left and right allows room in the middle of the mix for things like the drums, bass, and vocals which are usually best when left in the center of the mix. I usually like to leave bass and drums dead centered and single tracked (with heavy compression, usually with an 8:1 to 10:1 ratio and gain reduction of 5-10 dB, with make-up gain on to furthur control the output levels). My vocal tracks are usually doubled and panned slightly (5-10) both L and R.
The key to doubling effectively is firstly duplicating the takes as closely as possible and realizing how applying compression, modulation, and EQ to each of the different tracks changes the result in your mix. I like to take the two doubled tracks and pan one L and one R. Then I group the tracks together. Usually the first step after I record the two takes is to apply compression to each of the guitar tracks indivually. For this purpose I use moderate compression with 1-2dB of gain reduction and a ratio of 3:1 to 4:1. Make-up gain can also be used to assist with the gain leveling for the channel. I always apply a high-pass filter to guitars and usually set the cut off at 90 - 120 Hz. After compression and EQ is applied, I move to work on the group instead of the individual tracks.
When I move to the group tracks, I first play the section and watch the two halves of the stereo meter, watching carefully for the balance of output. The two tracks will not be completely the same throughout the section, but their indiviual faders need to be adjusted until you feel like the two tracks are "trading" back and forth at nearly equal volume throughout the piece. When this has been achieved, I compress the grouped track, this time with only slight compression of about a 3:1 to 4:1 ratio and only around 1-2dB of gain reduction. The fader will be used for further leveling and no make-up gain will be used on this compressor. After the final compressor on the group track, I will then EQ again. On the group EQ I will usually pick out one to two harsh frequencies which will be accentuated as the two tracks sum together. These frequencies are then reduced by 1-3dB with a high Q. I prefer to cut problem frequencies rather than boost the good ones. If done correctly, this will allow the guitar to remain present in the mix without being too loud. After the EQ other effects can be added depending on the style of the song. Usually I will add saturation to taste as well as a slight Large Room or Hall reverb. I find that the use of a saturator ( I really like the Brainworx Black Box Analog Design HG-2) allows the transients of the guitar to really become defined and stand out. And the slight use (5-10% wet) of a large reverb allows the two tracks to resonate together before being sent to the Master, allowing them to seem more like one part.
Hopefully this will give home recording artists some new ideas on how they treat and use double tracking. I would love to find out more about other techniques for double or even quadruple tracking and tricks that others may have found to help tweak their mixes and double track more effectively.