The ultimate productivity boost for a computer user would come from splitting the keyboard into two ergonomic pieces that each functioned simultaneously as a mouse. The button press for the mouse pointer(s) could be a universal downward push of the entire carriage assembly using the base of the hand, leaving the fingers free to type on the keyboard keys.
It would be difficult at first to slide around two mouse devices while also typing on them, but you could get it with some practice. Importantly, if kids were trained on it from the start, their amazing brain plasticity would adjust to it in a flash.
In addition to productivity increases, some cool new functions could be developed with two mice. For example, you could have a command to "throw" a piece of data or a window across the screen from one mouse pointer to the other. You could also do dynamic manipulations, as on the touch screen of an iphone where you "pinch" a map to zoom in or out.
IDEATION: September 25, 2006.