This is the only product that I think does what you are asking.
You might also look at Teleport - control multiple Macs using a single keyboard and mouse
While this is not dual monitors, it is the ability to run different apps on each machine, only need one keyboard and mouse.
I use Telrport between my 27" iMac and my Macbook Pro. There is copy and paste support between the two systems.
Thanks Bob. Will checke out ScreenRecycler.