Can you find (or write) software that simply sends and receives data via USB at each computer? If you can gain control over such communication then you might get a cluster concept to work, regardless of what OS they use.
Then you can add routines that make them cooperate at doing whatever operation you wish. Example, to make them play chess, you cannot use a normal chess program where you are a player. You need to get access to the software routine that awaits user input, and write routines which send signals back and forth between the computers.