I shan’t pretend to understand why you want the lower half of the piano keys sound from the left speaker, and the higher half of the piano keys sound from the right speaker - real pianos don’t do that. The high notes and low notes of a real piano comes from the entire instrument, not from one side or the other. Since the mid 19th century it has become usual for the low strings and high strings cross over each other inside the piano.
I’ve written a plug-in to create a stereo effect from a mono source - it won’t split the keyboard into left and right, but it will give a sense of three dimensional space to the sound: Pseudo-stereo effect