+1 to actually physically double tracking.
But in a pinch you can take the copied track and slide the waveform by a few ms. That will widen out the sound. It will completely collapse in mono, but it will make the sound wider and sort of fake the "double-tracked" sound. Also if you EQ the plugins a bit differently or use a different amp sim it will help. Or even detune it a bit.
I have done this to make rehearsal recordings (with only one guitarist) and such sound a little better. Or when mixing, and move a lead-ish rhythm part to the center and use the other rhythm guitar track on the other side. Or use a different repeat of the riff.
So you can fake it and make it fairly convincing. Doesn't sound nearly as good or as easy as double tracking.
Interestingly, the tighter you are, the better the sound, but no human can play it exactly the same way twice. Which just makes it that much more awesome!
You're right to a degree about it widening the track, but it won't "make it fairly convincing". It will still sound like one track, dead center, but just spread wider. It won't sound double tracked and won't sit well in a mix at all if using this method on rhythm guitars. It sounds bad.
However, doing this for a single take guitar solo/lead that you want a bit wider does sound pretty good. And if you got $60, you can get
Waves Doubler that works pretty well I've heard. (but it doesn't sound good for rhythm)
Remember, you have to make "room" for everything in a mix. If you have everything in the middle nothing is going to sit well in the mix and it's going to sound very muddy and jumbled.
There's no hard fast rule to it, but generally speaking:
- guitars are usually panned far L/R (I usually do 100/100 if dual tracking, and 100/100 & 80/80 if quad tracking)
- bass guitar is centered
- kick drum centered
- snare slightly R (maybe 10% R)
- rest of the drum kit panned as if you were looking at the kit (from an audience viewpoint) to get a natural feel
- leads usually centered or slightly panned if double tracked for a "wider" feel
- vocals centered or panned a bit for background vox/harmonies
Also adding a reverb buss for various tracks will give it a more live or 'wide' feel depending on how much you add.