Commercial servers have redundant everything, so that no matter what happens or fails, the server keeps on ticking. They're usually hot-swappable, too. A server we recently installed had (IIRC) dual hot-swappable PSUs, 3 hot swappable PIII CPUs, about 2GB of hot swappable RAM and dual RAID arrays, each of which could remain operational with individual drive failures. (Drives, of course, hot swappable!) The LAN connection was made to multiple backbones, in case those failed. Oh, and it was powered by two UPSs fed from seperate power sources.
Most redundant PSUs are just that: redundant. I don't think you can double the output, although I really don't see a reason why you couldn't... I've never messed with it so I don't know. You don't typically overclock a system that needs redundant PSUs
Pentium IV 2.4C Northwood- M0 stepping @ 3.2GHz HT
Air Cooled w/ SP-94 heatpipe & 92mm Vantex Tornado at 45C
4x256=1gb dual-channel 2-3-3-6 OCZ PC800DRAM @ 890MHz 3-4-4-8
MSI Neo2-PLS w/I865PE & SATA @ 4x267=1068MHz
BFG GeForceFX 5900@485/967MHz = 6284 3DMark2003s; dual-head 21" CRT & 19" LCD