Technicality question here --

If you set up a machine for VMWare, esentially having two identical Windows OS's on the same box, how does that affect licensing? Do you have to have two licenses since it's two instances of Windows, or does the same license work on both instances since it's still on one box, one mobo?

I know it can be easily done, just want to make sure it's legal. And yes, I know I can do Windows/Linux too, but I was just wondering about the legality of a Windows/Windows setup.