No, you can't just plug the second processor and all will be well. I know because i have tried it myself. The reason is that the HAL (hardware abstraction layer) in w2k needs to be updated.
What you have to do is either rebuild w2k from scratch or use the w2k resource kit. I don't know if updating the ACPI driver will work, becuase i didn't try it (i didn't know about it at the time).
However, i do know where you can look in w2k and see if w2k is using both cpus properly -
GO to system properties, click on the advanced tab, then click on the environment variables button. Under the system variables bit, there is a field called NUMBER_OF_PROCESSORS. Obviously it is going to be 1 or 2. If it says 2, then alls well. If it says 1, then you will have to do what i mentioned above.
Good luck.