iTunes Home Sharing now works between users on same computer - https://discussions.apple.com/thread/3865597
You could set up two completely separate libraries and share music via Home Sharing.
iTunes: How to share music between different accounts on a single computer - http://support.apple.com/kb/HT1203 - relocating iTunes' media folder to a shared area but leaving separate library files - extra tip at https://discussions.apple.com/message/17331189
Chris CA's instructions on sharing one iTunes music library between multiple user accounts - https://discussions.apple.com/message/8974074 - Multiple users using a single library file - similar post at: https://discussions.apple.com/thread/3753008
Suggestion by Turingtest for multiple users and one library - http://discussions.apple.com/message/9117622 - use smart playlists and tags to isolate groupings.
Discussions on using purchases from multiple AppleIDs in one iTunes library - https://discussions.apple.com/message/19543804