Nov 2010 - Ken Rushton
Rationale: key-to-note assignment can be made symmetrical, so one can transfer skills between hands, halving the number of fingerings to learn.
Each keyboard assignable to a unique instrument
Each keyboard assignable to overlap the other to a variable degree, making "special" effects like contrapuntal motion simpler, even trivial
One's hands won't run into each other
Ends the tyranny of the right-handed keyboard on the left-handed
That's the Theory, anyway. What it the actual practice?
It works, better than hoped for (I had wildly hoped for 30%). The Actuality:
There seems to be at least a 50% reduction in learning time for the left hand
An unexpected (but obvious in hindsight) thing that happens is clumsiness transfer from the untrained hand to the trained hand. There are techniques for reducing this.
I suspect some serendipitous (unexpected good fortune) benefits as well ... to be detailed.