I recall once seeing one of the Novell servers at the local Univ powered by two desktop supplies duct-taped together, hanging out of the open case, because the original one had gone south and they needed to keep the machine up waiting for the replacement.
The biggest question when dealing with multiple supplies is...where the heck are you going to PUT the second one...?
If you have an outsize tower, a Baby-AT or standard form factor ATX supply might fit into two drivebays...but it's going to look like crap.
I agree with what others have said above...just get a single stronger one, but make sure it's a *good* one, not a $19 special from the monthly computer show at the convention center...
One of my computers has two supplies because it's two physical units - a tower for the brains, and a separate one for some of the drives. The machine has 5 SCSI busses on 3 controllers
