This is an application of the Blumlein stereo technique (sometimes called crossed figure-8s, because the microphones are angled off the center axis by 45 degrees each and the sound pickup pattern is from both the front and back of the mic - a figure 8). This technique is usually used in front of a soloist or ensemble, especially when trying to capture more room ambience. However, in this case, we place the soloist in "front" of the array and the piano "behind" the array. The effect is similar to 4 channels of audio (left and right for the soloist/left and right for the piano) but with only two microphones to minimize phase delay and sound bleed problems. This also allows for more control over balance, as the musicians may move closer or further from the array to achieve the desired blend between instruments. Both instruments retain a crisp sound, as if individually miked. Also, because of the stage set-up, the mics are surrounded by much more stage floor (wood) than hall floor (faux marble). Wood tends to provide a warmer ambience (something that usually helps digital recordings).
