How can I share my iTunes library with my husband on the same PC?

There are two possible solutions, though neither is "easy" - iTunes is essentially based on a "one computer, one user, one library" paradigm, and it's facilities to share between users (Home Sharing, Family Sharing) are based on shared libraries being on different computers.

Your options are:

• Share one library between user accounts - typically by having the iTunes library folders in the Public user area, and using SHIFT-start to select that library when running iTunes. In this case both users will have the same library content, playlists, etc.
• Share one set of media folders but have separate libraries. Each user can then have distinct playlists, ratings, etc. but you need to recognize that if one user adds something to iTunes that content is not immediately accessible to the other - the second user will have to add the same media to his/her library to be able to play it, sync it to iDevices, etc.

In both cases only one user account can be running iTunes at one time.

I agree that this is a unfortunate limitation, but I would not expect any changes to iTunes to make it any easier. The scenario of two or more users sharing one computer is becoming less common ... indeed, I suspect that having more computers than people is now more prevalent.