Place the two microphones on separate mic stands and get the vocalist on a chair with the guitar.
Position one of the microphones so that the performer is singing into it at a slightly upward angle and the bottom of the mic is pointing directly at the guitar.
Take the other microphone and place it in front of the guitar. The mics should now be in a configuration that looks like a right angle.
The singer should be facing towards the left vertical axis and the guitar below the bottom horizontal axis.
Now create new tracks for both mics respectively. Assign one to the vocal and another for the guitar and record the performer for a few seconds. Maybe four bars of the song.
You will now notice that the bleed from the vocal into the guitar mic and vice versa are minimized. This will give more control over the performance and yield much better results than putting the performer in a less comfortable situation.