I accidentally used the same product key for two different copies of Windows 2000. Id it possible to change the Product Key?

Personally I wouldn't worry about it unless you have a pressing reason to do so. As long as you purchased two copies you are legal. It is not uncommon for example, in corporate environment to use the same serial number on all installations. It can be a hastle to always try to find the 'correct' serial number.

Go to Microsofts site and search for a utility called SysPrep. After you run this you will asked to enter a new serial....I think...it's been a while since I used it.

The Product ID is in the Registry under:
MyComputer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion and can be changed using Regedit. You should only use Regedit if you know what you are doing because editing the registry improperly can mess up your system royally.

Sysprep is for the deployment of cloned systems I have not used it on an individul computer so don't know how it works. It is included in Deployment Tools on Windows 2000 SP2 CD or can be downloaded from http://www.microsoft.com/windows2000/techinfo/planning/incremental/sysprep11.asp.