If you need some separation for mixing purposes while maintaining phase coherence, two figure-8 microphones may be placed close together and at a 90 degree angle, pointing towards the singer’s mouth and guitar’s neck/body area respectively - this will usually achieve a high degree of separation. Room acoustics will determine how well the technique works, as the figure-8 pattern will pick up the sound of the room. Some slight bass roll-off may be necessary as low frequencies may be accentuated by the close mic positions. Of course if you have plenty of mics, then you can get ultimate control over the sound by close miking the vocal with a standard large diaphragm condenser, and using one of the other techniques previously described for the guitar.