The Printer Sharer will allow two PCs to share one parallel (Centronics) printer without manual intervention. Users simply print from their software applications as usual, and the printer sharer lets the data through on a first-come first-served basis. The computer that comes second receives a "busy" signal until the first computer has finished. Most software, including Windows 3.1 Print Manager, will happily wait for the printer to become available.
http://web-ee.com/schematics/electronic_projects/printer_sharer/