Logitech's Unifying technology is awesome. Plug the USB receiver in the 
computer, power on your keyboard or mouse and it is ready for use! Well, 
the devices themselves do work, but software to attach new devices to a 
single receiver (or detach bound devices) is only available for Mac OS X 
and Windows.

ltunify is a program resulting from the gathered knowledge on the Logitech 
HID++ protocol. It allows you to pair additional devices like keyboards and 
mice to your Unifying receiver, unpair existing devices and list information 
about connected devices.

*** IMPORTANT *** - READ THE DOCS in /usr/doc/ltunify-0.2

Be sure you READ the information at the TOP of the 
/etc/udev/rules.d/42-logitech-unify-permissions.rules

In /usr/doc/ltunify-0.2 be sure to read the README.txt
for additional instructions on pairing.

If you need to always have one device paired and want it
paired on startup, place these two commands in rc.local 
exactly as you see them in this order;

ltunify unpair 1
ltunify pair