To configure a secondary site collection administrator for an individual OneDrive for Business site, follow the procedure given here:

1. Navigate to the SharePoint admin center Classic Features administration (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, as shown in the following screenshot:

5. Hover over the account name, select the down arrow that appears, and then select Manage site collection owners. This is shown in the following screenshot:

6. In the Site Collection Administrators box, enter a name and click Check names, and then select OK, as shown in the following screenshot:

The settings take effect immediately. The added user should now be able to access the site.