Warning: It has been many years since I touched this. I don't know if the USB host shield is still available or is still compatible with this code. Also, the old shield and software doesn't seem to work with the latest foot pedals from Kinesis. I advise looking for other solutions.
Arduino sketch: USB Keyboard combiner.
Detailed instructions: https://github.com/davidbkemp/KeyboardCombinerArduinoSketch/blob/master/Instructions.md
It combines the inputs from up to three keyboards, primarily to work around the way Mac computers do not allow the modifier keys of one keyboard to act on the keys of another keyboard. In particular, it allows you to use foot pedals as Shift/Alt/Command/Control modifiers for a keyboard.
Runs on an Arduino that can act as a USB client. Note that this limits the models of Arduino that may be used. I have tested it on the Leonardo and Due. It will not work with a Uno. It does not yet work on an Aruduino Zero as the USB Host Shield library does not yet support the Zero.
Requires the USB Host Shield 2 from TKJ Electronics. http://shop.tkjelectronics.dk/product_info.php?cPath=22&products_id=43
Relies on the USB Host Shield 2 library: https://github.com/felis/USB_Host_Shield_2.0
This sketch was adapted from the USBHIDBootKbd example in the USB Host Shield 2 library.
The code is released under the GNU General Public License.