iPhone cloud backup
Can I separate iCloud backup storage by device? In other words I would like to backup my wife's phone separately so when we purchase a new iPhone she'll only get her contents.
iPhone 7, iOS 10
iCloud stores iCloud Backups separately, even if multiple devices are using the same Apple ID for iCloud.
However synced data (the options at Settings > [username] > iCloud such iCloud Photos, Notes, Contacts, etc) are merged for all devices using the same Apple ID.
For that and other reasons it is highly recommended that each person have their own Apple ID. Many things can still be shared by using Family Sharing. Family Sharing - Apple Support