Adding devices

When adding the 2nd Apple ID to your Mac, unless you need to use both IDs in the same user account, create a new user account and add the second Apple ID to the new user account. To use both IDs in the same user account, you can sign into different places, such as the App Store/iTunes with one ID and iCloud with the other ID. Using 2 IDs in one account can make things more difficult.

User Account – Add New Admin.    -

User Account – Add New Admin In Single User Mode.  -

User account add  -

User account add (2). -

If you want your iPad and iPhone to sync with your computer, use the same Apple ID on the devices that you used for the computer.

As mentioned above, you can try setting up Family Sharing between the 2 IDs.

Family Sharing.   *

ASC YouTube for Family Sharing

Family Sharing - Set up.  *

Family Sharing - Set up on iPhone.  -

You cannot merge apple ids. You can use only one Apple ID per device. You can family. share between the accounts tho