Well one thing it could be is somehow the SLI configuration and KVM protocols are conflicting with each other, so much so that it is repeatedly sending data from the KVM switch back to the PC, hence why you are getting repeated presses.
Is the KVM switch connected via the PS2 ports? If so perhaps look into updating your motherboards chipset drivers. If its USB then it shouldn't be happening since you had a USB keyboard attached with no problems.
Also you may want to check your Nvidia's control panel settings especially for the SLI settings, just to make sure they are set correctly, try default settings if you are using your own manual settings. And if you are Overclocking the GPU try putting it back to normal.
Also look into trying out Nvidia Inspector
Link it is more informative than the Nvidia Control Panel and you get set things more precisely, game settings, Voltage settings etc.
EDIT: So was the KVM, maybe as I said try looking at updating your chipset on the motherboard, that may help.
Strange that by SLI'ing your cards it knocked out your KVM switch, unless they share the same IRQ.
You did uninstall your GPU drivers before installing the new GPU and SLI strip? Then reinstall them?
[Edited by DABhand, 7/13/2011 2:11:24 PM]