I connected three extended keyboards together. I wrote an app that, when invoked, caused the leds(3) on the Apple Extended Keyboard behave as described in the following manner.
All the keyboards would have their keyboard lights cycle through the octal codes from 1 to 7.
After the first keyboard cycled through the range of octal values and reset to 1, the second would change its state from 0 to 1.
When the first keyboard had cycled enough times for the second to cycle to 7 then 1, the third keyboard changed the state of its lights from 0 to one.
With three keyboards the cycle rate was low enough on the third keyboard so that it was clearly observable.