This is a fully functional keyboard switch (KVM but while only the keyboard),
based on the combination of Arduino UNO + Arduino LEONARDO pro Micro.
Arduino UNO sends data from a connected USB keyboard via SoftwareSerial to 2 Arduino LEONARDOs that emulate the keyboard.

Keyboard is toggled with ScrollLock Key.
I recommend USB cable keyboard.
All SoftwareSerials has baud rate: 38400

This is beta version...