Depends on the source, somewhat, but generally when people are talking about double-tracking (I assume that's what you mean by dual track), they're talking about two seperate performances.
Using more than one mic on a source would be done for two reasons - one, because you want to mix it in stereo and having different mics in different positions capturing different timbres can help increase that feeling of space. Or, two, because you want to treat the results as effectively a "mono" signal, but you want to flesh out the sound of one mic by using a second mic to capture some stuff the first one is missing (similar to the idea behind stereo micing, but trying to capture different tones, and not to pan them seperately to make it sound more spacious, left to right).
The former is something you'd do a lot on acoustic guitars - using either a matched pair of condensers, or like a small diaphragm condenser for brightness and a large diaphragm for body, and then panning them separately so make a big, lush stereo space. The fact that the mics will likely be slightly different distances from the source can potentially cause phase problems if collapsed into mono/panned together, but can help create an illusion of even more space and depth.
The later is typically done on electric guitars, usually a SM57 and something else (a Royer 121 ribbon mic or a Sennheiser MD421 dynamic are popular here, though some times you'll see a large diaphragm condenser put to work like this), where you'll have one of the mics (typically the SM57, which is a classic on guitars for a good reason - it sounds awesome on them) producing the "main" part of the tone, but a second mic recorded as well at the same time, and mixed in with the primary mic to fill out the sound a little - either add some more depth, or some added high end, or whatever. The idea is to make the single take you've recorded sound richer and fuller, and not to make it sound double-tracked. Often, you'll then go back and double track that first performance (captured with two mics) again, to get that "double tracked" sound.
Personally, recording instrumental guitar music, I don't necessarily WANT huge sounding guitars, since the guitar is both the rhythm and the lead instrument all at once so I don't want the two parts fighting each other for space. So, I usually just use a single mic, and double track my rhythm tracks (but not the leads).