There are different styles of KVM switches: most of the ones you find these days are powered, and you enter a key sequence to change among the computers. Some, if you can find them, have an N-way (e.g. 4-way) dial on the front to change among computers, and don't have any electronics to intercept any key sequences. (A downside of the non-electronic KVM switches is that some older OSs (Win98, most Linux setups) "lose" the keyboard and mouse when you switch away from them and switch back. WinXP recovers, though.)
The main difficulty in cascading electronic KVM switches arises as it becomes difficult to enter the right key sequence to address the second switch. If you cascaded one manual KVM switch to switch one or more different electronic KVM switches and computers, you might have better luck.
As a secondary consideration, I don't know how much power the electronic KVM switches draw from the keyboard and/or mouse connections, or how much a computer is willing to provide. There's certain to be a limit above which things just won't work, and cascading the switches only adds to the power draw.