Q: Why do I have to record acoustic guitar twice?
The question came from someone who had watched a video, presumably on YouTube, where the presenter recorded an acoustic guitar, then recorded it again exactly the same. Following the time-honoured tradition (or hoary old cliche if you prefer), he panned the tracks hard-left and hard-right.
In some senses this is an easy way to record acoustic guitar. It nearly always sounds better than a single guitar, unless of course that single guitar has been really well recorded by a great player with a first-class instrument, situated in a room with good acoustics, supported by an engineer who really knows his or her stuff. It does sound tiresome if every track on an album has twin acoustic guitars, but for one track in three it's fine.
Going back to the original question, I was asked this - "Why can't I just record the guitar once, duplicate the track, then pan the two tracks hard-left and hard-right? Won't that save me from having to record the guitar again?"
Actually it is a good question. To make the twin guitar trick work, the two guitars have to be very well in sync, and getting that second guitar to really lock in with the first can be very difficult and time-consuming.
But try looking at it this way...
You could record just one guitar and leave it panned centre. When you play it back the sound comes out equally from both loudspeakers and is in mono, because there is no difference between the channels.
If you duplicate the track and pan hard-left/hard-right, then on playback the exact same audio comes out from both loudspeakers. Since there is still no difference between the channels, the sound is - again - mono. So precisely nothing has been achieved.
There are ways to fake a double track. You can for instance duplicate the track and put them both through a very short delay of the same duration. Then time-modulate one of the delays. This works passably well for some purposes, but nothing is as rich-sounding as a second guitar track, recorded for real.
One last thing - The original question started, "Can't I just?". Well in audio you can't 'just' anything. To achieve a satisfying result, everything you do has to be crafted with as much skill, care and taking of pains as you can muster.