Do I need a stereo digital microphone?
What are the advantages of using a stereo microphone on the quality of sound? 


The advantage of a stereo microphone (or two mono microphones) is that it is possible to record in stereo for reproduction of a stereophonic image with sound sources located in the audio panorama. There is no advantage in fidelity of signal or signal to noise ratio.


  If necessary, how should I position these two microphones?


Position them to get the effect you require. This is a broad subject so I will not attempt to answer it here as there is plenty of information on the web.