Using two mics to record piano sound is typically done during live show. However creating sampling library isn't that simple. I've seen some people doing recording to high end sampling sounds. It wasn't a piano, but sound used for SF laser gun. They had a couple high end mics attached to the cable of a power line pole, and hit the cable to create the sound, then took it back to the lab to mixed them. Creating good piano sampling is similar, the engineer will use multiple mics some attached to the section of strings say A1, some mics on the harp and some on the box and some from far away... The sampling rate is usually very high, like 192k. The sound samples will then mixed in lab, and uses those to extrapolate the neighbor steps. Then samples will be taken from some other section of the strings to avoid over extrapolation. After final mixing, it will be converted into the sample files with lower sampling rates using dither.