it doesnt. i have a couple recording apps and none record in stereo. i dont know if there is an api open to use both microphones in the iphone 4
I thought the 2nd mic was used for noise cancelling only.
So with only one recording you will not get stereo.
the second mic is used for noise cancelation. but i thought maybe while u were recording in a recording app apple might open up the other mic as an api. they did for the flash as a flashlight.