Just wondering if this could be perhaps implemented. When set, there’d be an area where only the lower layer is played, an area where only the upper layer is played, and an area in between where both layers sound together.
To set up:
either hold “split” and press two notes together, the release the button; the two notes mark the first and last note of the overlapping section;
or press “shift” then hold “split” and play one key then another key, then release the button. Again, the two pressed keys define the first and last note of the overlapping area.
The latter implementation idea is perhaps preferable as it would allow for an overlapping area which is wider than a user’s hand span would allow.