Dual-Screen setup across two systems

As my current computer setup is falling behind latest game requirements, I've decided to build a new gaming-specific rig, which will have SLI support. I'll be keeping the current computer for school/work/storage/daily use, as it works fairly well for those tasks. I'd also like to switch over to a dual screen setup. My current PC only has one video card. I'm looking to satisfy the following scenario:

- Work PC will be setup on dual screen
- Gaming PC will be setup on one screen
- Both PCs will work off the same keyboard/mouse setup
- Work PC can still function when Gaming PC is active

That is to say, I'll have a main monitor which can switch between Work PC and Gaming PC, and a second screen which will work only with the Work PC.

How would I accomplish this setup?

Thanks Wolfshadw, that makes a lot of sense. Now let's say I want to take my setup another step further, and want two combinations of keyboard/mouse.

Let me explain. I'll set the KVM to the primary monitor, keyboard, and mouse to share between the work computer and the gaming computer. The secondary monitor will be dedicated to the work system strictly. If I switch the KVM to turn on the gaming rig, I still want the ability to use my work system alongside the gaming rig ... but I'll need another keyboard and mouse. I'm thinking I have to get two KVM boxes, one strictly for video, and another to handle the mouse and keyboard. That way, I can have the following scenarios:

- Dual screen work set up
- Gaming computer on one screen
- Gaming computer on one screen, work computer on secondary monitor