Piano filling both systems at once by defining a (variable) split-tone

Entering chords and melodies via MIDI-interface - e.g. keyboard - could be simplified for all keyboard instruments (piano, organ, cembalo, Celesta, ...) by placing all notes below a "split"-tone to the lower notation system, all higher notes to the upper notation system. The split-tone itself could be defined by a slider, placed over the virtual keyboard. The typical default would be the c'.