Seeing this software work was almost as cool as the first time I heard an 8 bit soundblaster make a voice from a computer..
It's called Synergy 2, you just install it on both PC's, tell it which monitor is where, and it acts as a KVM over IP. You just move your mouse to the side of your monitor (towards your other monitor) and when you cross the plane you're on the other PC, both keyboard and mouse, instantly.. No switches, no hardware, just blam works. So fast and easy to use many PC's at once now. I have it setup with my laptop (vista 64 bit) and one of my PC's right now.. Works like a charm. Also works over wireless
It's not meant for dual monitor on the same PC, but having seperate PC's with seperate monitors. My friend has it setup with a mac box, windows box, and unix box and everything works great between them.
http://synergy2.sourceforge.net/