Can I have two mac users share one iTunes library on the same mac?
It is a tiny bit ugly - but there are formal steps to accomplish exactly what you wish. Home Sharing doesn't work so well when both accounts have completely separate libraries - each in the usual home folder - so that's not a good thing to try.

The best workaround is to agree to store the media in a shared folder. It's more work to add the same music to both libraries - but you both have totally independent backups, playlists, play counts, ratings and apps.

This works really well and you can easily recombine later or split her or your account to another mac or PC any time you choose by taking a copy of the reference files as well as the library organization files.