This feature requires the multisite addon.
When you are logged into either of your websites, the sites where you have direct admin access are available in the top gray admin bar under the “My Sites” dropdown.
Why are all sites not showing in the dropdown?
Super administrators should be aware that only sites where they have administrator access specifically assigned will show up in this list.
As a super administrator, you have access to all sites regardless of your specific permissions on the website in question.
Just go to the log in URL for the site that you want to access.