QUESTION:
Can one assume that N+1 coverage when two power supplies are installed for 
an E450 that does not require 2 power supplies.

example: If you have installed less than 4 CPU's and less than 5 disks?

ANSWER:
Yes.  All three power supplies feed a common internal system power distribution 
board.  The power load is shared equally among all supplies installed.  If the 
total load is less than the output of a single power supply, then a second 
supply is n+1 redundant.  If one power supply fails, the other simply satisfies
the total power demand up to its limits.