Are you refering to CLUSTERING where you have mutible systems conected together (usually by ethernet) but running a single operting system?
Computer cluster is loosely coupled system.
I want to create a tightly coupled system. Consider the example of IBM Blade Server.
I want to implement similar concept using old motherboards.
The IBM blade center contains up to 14 separate servers connected to 2- gigabit Ethernet switch as the primary interconnect.
The system uses 4 common power supplies, 2 blowers and 2 Chassis management modules to allow for remote management.
The backplane provided the electrical interconnect for the devices and a private 100m Ethernet management channel to the components.
A secondary fabric could be used to add 2 additional switches for Gb, Fibre channel or inifinaband.
If your trying to mimic the interconnect on this system, all you need is a simple dual gigbit network.
The blades were frequenly used indivulally as part of a IT infrustucuture. One for DHCP, one for mail, one for SAP, etc.
Or as parts of a standard cluster.
This link will take yo to a Intel white paper on High Availability Server Clustering Solutions