hi stanley,
after you create the dba database app and double click it (setting gear>site content>find dba database app), can you get the page shows as below?
if you click open this app in access to start adding tables, can you import these two databases via clicking the access button at the bottom of the applications.
normally we use this method to import an access desktop database. for the split database, you can check if it works for both two databases.
if they cannot be imported with this method, you may need to find some other workarounds to share access desktop database (e.g. create an access web app and upload it to a site). to learn more, see ways to share an access desktop database. thanks for your understanding.
moreover, if it's convenient, could you share with us these two .mdb files for reproducing the steps?
to protect your privacy, i have sent you a private message to collect these two files. you can find the message via the url https://community.office365.com/en-us/user/conversations.
best regards,
tim