How to merge parts of two iPhoto libraries and remain separate users

Question

We have two user accounts - let's call them A and B - on our iMac and hence two separate libraries which is fine for the major part of our photos, but we have many photos of mutual adventures and hopefully more to come in the future. If this is relevant: the photos are imports from other computers, but are no longer available from these computers. There's a continuous backup on Time Capsule via Time Machine.
We would like to share the photos from the two existing accounts, except those that are from each users past.
I expect to have to move or copy some photos including new imports in the future to a new shared library AND keep our old libraries from the past separate as they are from the past.
I further more would like to share the photos from mutual adventures to to Apple TV - maybe via Airplay as I can already do with the iTunes library on account A.
It seems to me the best way would be to have a separate user account -  account C - for mutual iPhoto and our music in iTunes, BUT user A synchronize the music to my iPhone and user B does not yet. To complicate things - I think - user B has less storage on the iPhone than user A, so the full iTunes library simply does not fit on user B. I do realise it's possible to do selective synchronisation of music, but will this work in such a setup?
How do I do this?

Answer 1

Thank you for a quick reply. Unfortunately this is turning out to be more complicated than I initially had hoped. Please see why in explanation below.
I would like to give more information about the setup, just to be absolutely sure there's no smart setup for us. As user A was the first user in our network this user has an iTunes id which is used for the Appstore/iTunes syncing iPhone/Airplay to share music and photos from user A's library. User A (unfortunately?) also has an iCloud id which is different from the iTunes id in order to use the iCloud sync features for Mail/iCal etc. User B has an iTunes id and a small music library, but is currently not using it. User B also has an iCloud id to use the sync features.
From your answers it seems there's no way to "get it all" (sharing, syncing, personal/mutual data) in our current setup and the whole iCloud/Air Play/sharing is a little confusing to me, but given that the primary purpose is to show the mutual photos as a screensaver on our iMac and Apple TV and play the music there too there are two ways to do this:
1. your suggested setup with the extra library (to be shared via Airplay to Apple TV with an iTunes id assigned to the new Media user iTunes/iPhoto library). In this setup user B looses the ability to see photos on the iPad unless duplicates exist.
2. copying all mutual photos for user A respectively B to two separate albums in the existing separate libraries and making these albums available as screensavers and sharing via Airplay, BUT with the limitation that only one user's album will be available at a time as screensaver and as far as the Apple TV goes the users two separate iTunes id's have to be used to access the album from one user at a time.
Question:
Ref. 1 As I understand it iTunes purchases are linked to one id. So what are the consequences for a user if the iTunes id is in use for the Apple TV homesharing via Airplay and a logon is attempted on the user's iPhone?
Ref. 1 As I understand the sharing setup the iTunes id for user A with the most music must be the preferred one for the new Media user and the music library for user A must also be copied to the new Media user?
Ref. 2 As I said the whole iCloud/sharing/synchronizing issues are a little confusing. Given the above information with two separate iPhones, iTunes accounts, iCloud accounts and the wish to share photos/music
Do you see any smarter way to do this than your initial suggestion?