Mac Pro Tower, 1 or 2 CPUs
It will work, but it will make the CPU fan on the installed processor run at full speed. When I upgraded the processors in my '09 I made sure I had one running correctly before adding the second and discovered the issue. Fan noise is quite loud. You might be able to limit it with a third party fan control app, like 'Macs Fan Control', but I'm not positive and am unwilling to test it by removing one of my processors. :)

Also, you will be limited to the four RAM slots of the installed processor.