How do I create another library in itunes?

The best way is to create a new Windows user account, open iTunes there and let the new Library (for another user?) exist there.

To create a new user account in Windows, visit your Start Menu > Control Panel

You can transfer content to the new user either through a "shared access folder" or via a external drive.

I would copy it first and when it's successfully on the other user, then backup/delete the content from the first user.

You're correct. That does not work. Going to try ds store suggestion

If you're using Windows, look in your computer's Help documentation and learn how to set up additional user accounts.

1) Click the Start Menu, and then point to "Help" or "Help and Support."

2) Enter "new user" in the Search field and press Return.

If you use a Mac, read this article:

Mac OS X: How to Share a Computer With Other Users
http://support.apple.com/kb/HT2542