If you’re like me (and many of you are), you have two Apple IDs for historical reasons that you continue to use. Until the mythical future date at which Apple updates its back-end systems to achieve the glory obtained only decades ago by most businesses that manage accounts to allow merging purchases and other data from multiple IDs into a single one, we’re stuck with it.
(Sidebar: Many, many, many people write in regularly asking how to solve this problem, because they assume they simply haven’t found Apple’s merge-accounts options. It’s not you, folks.)
Years ago, iOS could interact poorly with two ID setup, but then Apple switched to embrace it fully. Now, in iOS 10, the Settings section for accounts even lists iCloud and Store as separate items if you’re using two accounts along with the email addresses for them. That’s a nice move.