Well considering I'm not a Windows user, I don't know if my suggestion will work. But in Windows, I know there is a "Shared Documents" folder where users can place files that they want to give others access to. I also know that with iTunes 7, you can choose which library you want iTunes to use each time you open iTunes. Just so I could tell you how to do this, I fired up Windows and downloaded & installed iTunes 7.0.1. To choose a different library when opening iTunes on Windows, hold down the shift key while you click on iTunes.
So User B could put their library into the sharing folder then User A could point iTunes to the sharing folder. When User A is finished using User B's iTunes library, he/she can quit out of iTunes then open again with the SHIFT key held down to choose their normal library.