http://hexus.net/tech/news/psu/99538...edundant-psus/

FSP has launched its new Twins Series of PSUs. As indicated by the name, these PSUs contain dual hot-swappable power supply modules. They are said to be ideal for startup companies' web servers, mail servers, or home servers where you wish to benefit from the security of two independent PSU modules in one. Furthermore the Twins PSUs allow such users to avoid upgrading to more costly PC form factors.

The two models from the Twins Series differ only in wattage output. As indicated by their model names they are 500W and 700W rated units. Each PSU contains two independent power modules with its own power adaptor. In normal operation the Twins share the load between their two modules (for maximum efficiency and stability) but should one fail "the other automatically takes over as a clean fail safe". Thanks to the hot-swap functionality you can then easily swap the dud PSU module without any downtime.