When the mics are spaced the stereo image is usually stronger. (Although it does depend on how far apart they are) But they may not collapse into mono very well. There could be some comb filtering and the guitar sound may not sound that great in mono.

XY or MS where the mic diaphragms are in the same spot will still create a reasonable stereo image but will not have any problems if collapsed into mono.

I have found that stereo acoustic guitar sounds are over rated and in the end you don't need it so much. By the time other stuff comes into the picture you wont hear it that much anyway. You never listen to an acoustic guitar 6 inches from the strings do you so creating stereo images is not that important. You tend to hear it further away and then it becomes mono anyway. Once other elements are in as well you find that the stereo image is more hassle than its worth. And the OP is already talking about narrowing the image anyway.

A much better effect is to get the acoustic guitarist to play the same (rhythm) part twice and just pan those parts hard left and hard right. And if they are good they will get the timing very spot on and it won't sound like two guitar parts but more rather like one.

One decent mic a little out from the 12th fret usually does the trick and is hard to beat. What is WAY more important is a decent guitar and player.

Thanks everyone for your replies. So far it was just a recording rehearsal and I was interested in trying different methods. (Will probably end up with XY technique though and make sure I also do a mono guitar recording at the same time to be on the safe side!)

It's a sparse folk  arrangement, close up intimate.  (one vocal (quite low male voice)), one guitar (finger picking/not strumming) (guitarist is not the singer) and violin. My plan is vocal centre, violin off to the right a bit and guitar over to the left a bit. I thought I should try some stereo guitar to help the guitar 'bloom' but still keep it authentic and not unnaturally wide (so the guitar isn't wider than the singer and violinist)

Would another option be to use eq and take some top off the left guitar (low pass) and eq the bottom off the right guitar (high pass) and pan the two mono tracks say left mono 70-80% left and right mono 0-10% right as if the guitar takes up most of the left soundstage? Therefore, the higher frequencies of the guitar are around the centred low male voice for more separation.

(I know that it's best just to listen! ... but I've been looking around the internet to see different acoustic guitar stereo recording techniques, but can't find much on how they are mixed thereafter) and would like to know how others in the forum go about this kind of thing. And ... I know ...  I'm wayyyyy over- thinking it.

@ gcolbert ... yes, sorry ... this should probably be in another forum.