XBoxLPU: a normal KVM will be able to do this for you. Set up your main system with one monitor connected to the KVM (the one you want to use on the second machine) and the other monitor connected directly to the main system's video card. Then connect the two systems' keyboard and mouse ports to the KVM, and the actual keyboard and mouse to the KVM. When you have the KVM set to the primary system, both monitors will display as a normal dual monitor. When you switch it to the second system, the keyboard and mouse will switch, as will the monitor connected to the KVM. The other monitor will continue to display the output from the first system with the dual-head video card, because it has no clue that anything has changed. Your video card still "sees" two monitors because the KVM maintains a signal link.
Again of course, you could do the same thing with just a VGA switchbox, which will be cheaper. Setup the output from one video card port to send to the KVM, the other video card port directly to the second monitor, then connect the other machine to the other switch port, and the monitor to the output on the switch. When you click it over to the second machine, the monitor will switch, but the second monitor will still be displaying from the first computer (and if you use a VGA switchbox, you'd have keyboard and mouse input to both machines, so you could work on them both at the same time when needed, and then switch back to dual monitor on the primary machine).