[caw]

For full redundancy, yes.

You get redundant power supplies by plugging both into the same power feed.

However, you can gain some extra redundancy by plugging each power supply into a different circuit, and reducing the common point of failure (shared power source).

[nickzoic]

Two UPSes.

Although I've got a good war story on that one too, I had a machine with redundant PSUs emit a stream of smoke from one of the PSUs. It was happily up and serving while smoking out the machine room until the operator (rather sensibly) grabbed both power leads and pulled, at which point both fire and ssh went out ...