I'm getting a socket 940 board soon. The board has 2 sockets, but I think it works with just one CPU plugged into the board. My question if there are separate rankings for 2 CPUs and 1 of the same kind (like 2x Opty 856 won't be compared to 1x Opty 856), so I can score twice as many cups by removing one chip and re-bench everything?

Not a chance mate, cpu's are seen differently here.

 

The only benches where you see a difference is in wprime and maybe 3Dmark06.

Wprime, 3dmark03 and 3dmark06 and not to forget, pcmark.

 

Dual cpu gives you with some benchmarks extra advantage