One, I don't know of any cases (outside of server mounts) that have space for more than one (1) PSU.
Two, most people just us a battery backup for power problems.
Three, if you buy a good PSU, it will fail gracefully and you just replace it (takes 5 minutes).
Four, for the price you'd pay for 2 PSU's, you could buy 1 good one that you won't have to worry about.