I actually have been using one photo library for both myself and my wife for 2 years.
The trick to this is in Settings on iOS to have both devices logged into the same account on the iCloud option. This account I use for purchases & backup (“Family Account”). So all of mine and wife’s devices are backed up here as well as having 1 photo library.
The next step is to only have turned on “Photos” and “Find My iPhone”. The Find My iPhone isn’t necessary for this to work but I keep it on for security.
Then with regards to the other data, use another Apple Account (each family member) for Contacts, iMessage, Facetime, Mail, Reminders, Notes etc. My wife actually keeps all of her contacts on her Exchange account from work, but I keep all of mine under a different Apple account using iCloud.
This will bifurcate all of the other data to the user’s specific account.
So the only thing that I see from my wife’s iPhone is her photos and vice versa as well as the backups.
This also allows her to remain under the iTunes Match subscription (added bonus).
The only issue I have found that is kind of annoying is that since there are essentially 2 Apple IDs logged in under 1 device is you will now see 2 “Family” lists in Reminders (each Apple ID that is logged in). As well as under Notes you will have both Apple IDs so make sure you change the description under the “Mail, Contacts, Calendars” option in settings to a unique name in the description or you will have 2 accounts named iCloud (my solution was to name 1 “iCloud-Family” and the other “iCloud-jz”.
The other part of this is you will want to make sure that the default account that Siri chooses to add Notes to and Reminders to is the individual account and not the “Family” account.
Thanks and love the show.