Start with the single point of what they want, in this case two servers to cover the lost off one, then explain that the likeliness of a power failure is higher than the server failing, that the AC failing is higher than a single server failing, that perhaps a tornado or other natural disaster is higher than the server failing. Of course you'll have to find all of the stats as they relate to you.
Then explain that it's pointless in covering the server if you don't take care of those things first.
I'm not sure where it came from but Scott said that Dell server have a 4 hour out of 8 year expectation of down time.