I have the same issue. I have a notebook and a desktop at work that are connected to different network and I would like to use a single mouse and a keyboard to control both of them.
I have found 2 solutions for it. First, there is a somehow expensive 23 Euro (some KMVs are less expensive than that) device on eBay that is exactly what you are looking for:
2 Port KMC21 USB KM Keyboard Mouse Switch ie3
Now, look at the issue there. You will have to press the button when you need to switch between the two.
My idea was more of having the continuous screen, so I could move the mouse from one device from another, without the need of pressing any buttons. The idea is like continuous dual monitor but with dual computers instead. For this task, I need to put 2 computers on the same network, so the data could be transferred via IP. I do not want to buy additional network cards to connect two computers, neither I want to use the wireless (which the desktop computer does not have, by the way). The only solution that I have come up so far is the USB-To-USB cable that works as a network bridge. There are only a few cables that emulate a small local area network (it should be labeled as supporting TCP/IP or LAN connections). Some on amazon say they do but they actually do not, check the comments if you will be looking for such a cable.
With such a cable and free software called Synergy,(
cnet video,
Synergy on Sourceforge), I am sure I will be able to make it work. So, now the only thing to do is to find the right USB-To-USB cable.
I will comment more when I will order/get the cable and make it work.