Q: I have two iCloud accounts, one for family and a personal one for myself. I want to be able to share documents and data between my iPhone and my new MacBook Pro with apps such as Pages, Keynote, and Numbers. I already have those apps on my iPhone that were bought with the family account and I want to sync them with the apps I plan to buy on my MacBook Pro. Do I have to buy these apps on my MacBook Pro using the family account, or can I buy them with my personal account and still have them synced with the same documents and data as the apps on my iPhone? On my iPhone I use the music and apps through my family account but everything else such as mail, contacts, notes, etc is connected with my personal account.
A: The short answer is that the account used to purchase an app has absolutely no connection to the iCloud account used to sync data for that app. iCloud documents and data are simply synced with the iCloud account you are using on your device.
When using multiple iCloud accounts, the main point to keep in mind is that the account you want to sync documents and data with—your personal account in this case—must be setup as the primary iCloud account on your device. Basically, this means that it must be the account that is configured under the main iCloud section in your iOS Settings app, and the first account configured on your Mac. The Documents & Data setting will only appear for the primary iCloud account; secondary iCloud accounts can only be used for Mail, Contacts, Calendars, Reminders, and Notes in addition to the primary account, and for Safari Bookmarks/Reading List and Find My iPhone instead of the primary account.
As long as the Documents & Data setting is enabled in your iCloud preferences, all iCloud-enabled apps will store their data in this iCloud account regardless of the account used to purchase the actual app.
Technically we’re actually talking about two completely separate “accounts” that simply use the same Apple ID and password: An iTunes Store account to purchase the apps, and an iCloud account used to sync your data. These accounts are not really connected in any other way beyond sharing a common Apple ID in order to provide you with a “single sign-on” experience. The same holds true for accounts used for FaceTime, iMessage, and Game Center—none of these have any inherent connection to each other beyond the Apple ID used to access them, so for example you do not need to use the same Game Center account as the App Store account that was used to purchase or download a given game either.