If you are running two separate computers as I am you could soon go crazy using two keyboards and two mice.
There are hardware solutions called a KVM switch (Keyboard, Video, Mouse) that allows you to use just one set to control everything, but these can be semi-pricey and you have to manually switch between what system they are controlling.
I didn’t need a monitor solution since I setup my main monitor to work with either system and could simply select the source at the press of a button. I did though need a good solution for the keyboard and the mouse.
Thankfully there is a great free open source software called Synergy that allows you to use a keyboard and mouse on one system to control another system via a network connection. Synergy is just plain great software and I use to use it with my main PC and a laptop. There are versions of Synergy for both the PC and the Mac and they inter-operate beautifully. Even better is that you can do copy/paste operations from one system to another. It is great to just move the mouse cursor from one system to the next and the keyboard automatically works with whatever system currently has mouse focus.
Lifehacker has some good instructions for how to set this up for a dual systems (specifically a PC and a Mac). The interface for setting up the PC version isn’t exactly pretty and a little confusing to get going. Though someone has created a GUI wrapper of Synergy for OSX that is much easier to setup.
Synergy once setup just plain works and you soon forget about it and just go to work using its capabilities.