I’ve helped a few clients through this process. Apple has always expected each person will have their own Apple ID, and as more and more of our Apple lives are linked to an Apple ID, it’s only become more important.
To answer your questions:
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You cannot share the same photo account (iCloud Photo Library). Yes, there are some aspects that allow you to share photos, but each Apple ID has its own iCloud Photo Library. Photo management is an ongoing challenge, and a definite down side to giving up on the idea of a shared Apple ID.
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Any data linked to that Apple ID will not be lost, but the moment you log out of the shared Apple ID on any device, you will lose your ongoing access to the data. For some apps, you may be asked if you would like to keep the data on your phone. If you choose to do that, it will be on your phone only – and it will not sync to your new Apple ID. That’s not good; one of the main benefits of having an Apple ID is seamless device sync.
So, you will have to export the data while logged into the shared Apple ID, then import it while logged into the new Apple ID. This is much easier to do with a Mac. I’ve done it many times for clients. AFAIK, there is no Apple tool to migrate content from one Apple ID to another.
How, exactly, you do this in a reliable and safe manner is too lengthy to describe here, but it’s not difficult, just time-consuming. Also, some content doesn’t need to be migrated but instead can be shared; for example, you may want to use shared calendars so you both see the same events. (Sharing calendars is sanctioned by Apple.)
- I’ve mentioned lack of photos sharing to be the biggest drawback, and behind that is Contacts. Like photos, Apple has no way to reliably sync contacts. One-time sharing of a single contact is beautifully easy, but it’s not sync.
Here’s the overview of how I approach this:
A. Go through all the apps that have iCloud data and determine what you want to have happen with that data. Don’t do this from memory; you probably have more data in iCloud than you expect. Make a list and note which Apple ID needs a copy of what data. Take notes throughout this process.
B. Find a Mac. (If you don’t have one, you can borrow one and delete the accounts when you’re finished.) Have one account on the Mac logged into the old (shared) Apple ID, and a new account logged into your new Apple ID. Export the data in the old Apple ID, move it to the account with the new Apple ID (the easiest and most reliable way is a thumb or other external drive), and import it into its associated app. Wait for sync to happen, and confirm that it’s synced to Apple’s servers by logging into icloud.com in a browser.
C. Once you’re happy that all the data seems to be in iCloud, then log into the new Apple ID on the iPhone or iPad. Confirm that it all synced correctly.
D. Delete any data no longer needed by any device connected to the old Apple ID.
This procedure is a one-time thing. A bit painful, but you won’t have to do it again and you won’t run into further problems.
There are other ways to move data around (via sharing), but I think this the most efficient way to “split” an Apple ID. It’s the easiest way to move lots of different data types without getting confused.
Please note that this is for iCloud data only; data linked to other accounts will be handled differently.
And also: if you’re adding/using several email accounts, note that they may come with their own Contacts syncing (and sometimes notes, but Contacts is the area that most people get into trouble with). I strongly recommend that you use only one service for syncing Contacts data unless you have a very good reason for it (i.e., required a work-provided account).
Finally, remember that you can stay logged into the old Apple ID for purchases (media and apps). You access this in Settings > Apple ID > Media and Purchases.