To get good separation when tracking a live acoustic guitar/vocal performance, you can use two figure-8 mics. Position the vocal mic so that the performer is singing into it at a slightly upward angle, and the bottom of the mic (the bottom null of the figure 8 pattern) is pointing at the guitar. Position the guitar mic slightly above and in front of the guitar angled down towards the strings, with the top of the mic (the top null point) facing the singer's mouth.
The mics will form an L with the singer facing towards the left vertical line and the guitar below the bottom horizontal line.
This technique works best with ribbon mics, as they yield much less off axis coloration than a condenser mic . This gives you a bit more freedom in positioning the mics for best separation without adversely effecting tone.