Recording acoustic guitar in stereo - should you use spaced or coincident mics?
It is often easier to get a pleasing recording of acoustic guitar in stereo than it is in mono. If the guitar will be an important component of your track, then it is certainly worth considering. But you will be faced with the choice of using coincident or spaced microphone technique. There is no one best technique for all circumstances, so you need to think about what you want to hear.
Coincident stereo microphone technique is where you use directional microphones (i.e. anything other than omnidirectional pattern) and place the microphones very close together, pointing apart. You can choose the purest form of coincident technique where the diaphragms of the microphones are absolutely as close together as possible. Or you can space the microphones up to around 10 cm or so. Try both to see which you prefer.
There is no hard-and-fast rule where the microphones are spaced so far apart that the technique is no longer coincident, but when the spacing is significantly more than 10 cm, you will hear a very different sound texture. You could space the mics by 30 cm and point them in parallel at the guitar, one on each side of the sound hole. Or you could space the mics by the full width of the guitar and point them inwards. The wider the spacing, the 'spacier' the sound.
Pros and cons of coincident technique
Coincident technique gives a very firm and solid stereo image. If the guitarist moves slightly in performance, the level in the two channels will vary slightly. The closer the mics are to the guitar, the more this will be heard. Usually however, any slight changes in level don't affect the overall sound too much. You can always ask the guitarist to keep still (and let them hear the problem so that they understand why they need to do that).
Coincident technique also provides full mono compatibility. So when your mix is eventually played on a mono radio, mono TV or center-cluster PA system, your guitar will sound exactly as good as it sounds in stereo, except it will be mono.
Pros and cons of spaced technique
The spaced technique creates a much more open sound. It is difficult to explain in words, but you will hear it immediately when you compare the two techniques."Wow - that is so much better" - that's what you'll say, and so will musicians and other non-technical people. What sounds better immediately might not sound better in the mix, so you'll need to think ahead.
There are three issues however...
The first problem is that when the player moves, the channels will not only change in level, they will change in timing. If the player moves from side to side slightly, then the guitar will move closer to one mic than the other, then vice versa. The result is a wandering stereo image, which can be very distracting.
The second problem is mono compatibility. A spaced-mic recording played in mono will often sound worse than it does in stereo due to phase cancelations. The trick is to find, by experience and experiment, microphone positions that sound not too much worse in mono. What you're doing in effect is improving the sound in stereo at the expense of the sound in mono. It's a compromise that will require a considered judgment.
The third issue is that when the mics are spaced very wide, the sound can almost become like two guitars. Now this isn't necessarily a bad thing. In fact it can sound very impressive. But you need to listen out for this effect, and ask yourself whether it is really what you want to achieve. If it is, then go ahead.
In summary, there is no right or wrong in coincident or spaced microphone technique, but you need to understand the issues involved and make sure that the sound you achieve in your recording is the sound that you want, and that it didn't just happen by accident.
P.S. You can also achieve a stereo recording by using a single mono mic close-to, and distant widely-spaced mics to capture the ambience of the room. Pan the close mic center and the room mics hard-left and hard-right as a starting point for your mix.