To configure a secondary site collection administrator for an individual OneDrive for Business site, follow these steps:

1. Navigate to the SharePoint admin center Classic Features administration center (https://admin.microsoft.com | Admin centers | SharePoint | Classic Features).
2. Launch the User Profile Service application from the classic SharePoint admin page by selecting User profiles.

3. Select Manage User Profiles.
4. In the Find profiles box, enter a username and click Find to search for them:

5. Hover over the account name, select the down arrow that appears, and select Manage site collection owners:
   

6. In the Site Collection Administrators box, enter a name and click Check names. Then, select OK:

Now, the added user should be able to access the site.