I want to do a side by side demo of 2 systems running a game.  I'd like to be able to have 1 keyboard and 1 mouse talk to both computers at the same time.  Not a traditional KVM.  I don't want to switch between the computers.  If I left click - I want both computers to respond.  If I type - I want it to show up on both machines.
 
Anybody know of a way to do this?

Why not just get two monitors?

I have 2 monitors and 2 computers.  I want the mouse and keyboard commands to go to both at the same time.
 
Do you mean 1 computer with 2 monitors?  I want to show the performance difference between the 2 computers both running the same game.

Not sure if that would be possible. Say one of the two computers runs a game less well than the other and say that the game lags on that computer. Wouldn't the games split at the point? Then you'd be controlling  a game at two parts of the game at once!

its very possible to do this
 
2 options:

Technical (requires scripting knowledge)
a) www.autohotkey.com
this software will allow you to customize your keys to do what ever you want. I use this program to play world of warcraft simultaneously on 2 seperate accounts. it requires some scripting knowldge.

b) keyclone - this program will allow you to set up your binds to use on a keyboard.

***you still need one other program (synergy) to "broadcast" the keystroke to both pc's at the same time***

More info at http://www.dual-boxing.com/forums2/viewtopic.php?t=6
Info2: http://www.dual-boxing.com/forums2 [...] php?t=1126

 
I don't expect to be able to play the game well.  Just a 30-second synchronized demo to compare 2 graphics cards.  I really just plan to enter a scene and look around.