Connect iCloud accounts
How do I connect iCloud accounts to 1 device??
Question marked as
Any device can only be signed-in to a single ID and there is no way to “merge” AppleIDs.
There are also limits on swapping a device “back and forth” between accounts.
Setting-up a “Family” of the two AppleIDs and then “Family Sharing” between them
is the closest approximation of what you’re wanting to do.
Set up Family Sharing - Apple Support