Plug single PSU into two different power sources

We have a rather uncommon situation: We have a server in a computer center. Its got not enough spare space to put in a redundant PSU. Nevertheless, we want to attach the single PSU to both existing power sources, because it happens fairly often (in terms of uptime that is) that one of the power sources goes of for maintenance.

Is there some sort of magic black box (almost like a UPS), that can handle two power sources to output a single power cable? Is there a PSU with 2 power inputs? Any other ideas? Did I miss something?

Please Note: The solution has to fit in a rack!

Thank you all already!

Solution 1:

A transfer switch will do what you want. APC's can be found here.