In late 1999 and 2000, I did some work with friends on setting up Linux PCs so 4 people could use one PC simultaneously. We had some successes, though there was a real struggle to implement that many PCI video cards on each machine. (Input was via USB devices.) Even so, we had 4 users humming away on one machine in a call center and in a charitable agency office where the only tasks were text input and other secretarial tasks and some database fetches from a server with, along the companion inputs. For all I know, they're still running.
Now, a company called "Userful" has software to manage things easily and reliably. No more custom setups.
Think about this for a minute. Not all, and in fact most users probably don't actually need an entire PC. This is ever more true when the desktop computers available to us are growing in power and capacity as fast as they are. Got 3 kids? Why do they each need a computer? Build one killer machine and implement Userful's stuff and you will probably end up saving money, trouble and stop that terrible hair loss that affects us older guys.
Give it a look.