I am writing this using two keyboards simultaneously, 10 finger method, both hands using a board of it's own. You can check which buttons have been pressed with which hand. So I guess this proves that a with a secondary keyboard the message bar must be off if you want to use it as a controller.
Nevertheless, I'd like to know how the wiring is done. All modern keyboards seem to have a three layer film system under the buttons, and although changing the buttons to switches is in theory a simple task, soldering the switches on the plastic film doesn't seem like one.