The frustrating fact is that sticking a microphone inside a guitar body is not going to deliver anything like the sound you hear in front of the instrument. It can't be. And even the best piezo pickups don't sound like a miked-up acoustic guitar. You can mess with it, you can EQ the crap out of it, still doesn't sound like a miked instrument.
The more mics you put in front of the guitar, the more bleed you get. There's no perfect way around this unless you use a single mic with a very tight polar pattern, and find a way to aim the mic at the guitar so that the mic's diaphragm aims away from your voice. This usually doesn't produce the ideal sound, but if you try it, aim for the point where the neck hits the guitar body and point the mic as best you can so it doesn't aim at you while you sing.
I
always record vocal and guitar separately unless the singer simply can't perform without playing the instrument while singing. I've only had this happen once and here's how I solved it:
I had the singer hold the guitar and make the chord shapes while strumming extremely softly in one pass, with one finger or the thumb, and then overdubbed the acoustic guitar without singing. Yes, you'll hear a bit of this strumming when you blend in the "good" acoustic track, but it beats a sharp stick in the eye.
However, let's assume for the moment that with a little practice you can deliver a separate vocal performance.
The setup you're using is a bit hit-or-miss in creating a stereo image, which is the whole point of using two mics, right? If you can play the acoustic without singing at the same time, try this setup:
I use a spacebar and mount the mics in a Nederlandse Omroep Stichting (Dutch Radio Foundation) 'NOS' stereo array. The right distance is when you hear the stereo image solidly snap into place in your cans. I sit on a rolling chair and move the chair around while I'm playing to find this distance from the mics, mark it, and then set up a proper stool so there's no wheel noise. With this technique and a couple of decent mics, the guitar sounds like it's live in the room. The imaging is fantastic.
Here's how to do the NOS technique, though it's for recording the instrument if you're
not singing, because there
will be vocal bleed:
en.wikipedia.org
Happy recording and good luck!