jmoose offered a very good explanation above. When recording my wife (who sings and plays acoustic) I set up two cardioid mic's and record a scratch track(s) to a metronome. Sometimes I even just take the piezo signal since its just a scratch track. Then we go back and replace the vocal & guitar takes one at a time. This results in a better take for her but can literally suck the soul out of the song for others. Some singer songwriters sound better singing with their acoustic and recording without a metronome. The two figure 8 mics is kinda a work around for the artist who has to record vocals while playing their acoustic. Since your just starting out you may want to get a single multi pattern LDC like a used AT at4050. This way you can explore some of the other patterns and maby add another one to experiment with the techniques above. But if you can record the vocals separate it will allow you to get a more traditional lead vocal sound.