It is now possible for a user to be a TA and also be a member of a Workspace with a single login.
The process of promotion to TA process is described in this article.
Understanding the permissions of a TA
As a Tenant Administrator, you can add, edit, hibernate and wake up workspaces. You can also manage users and their Workspace memberships. If you are added as a member of a workspace then you may loose some TA privileges for that Workspace as the workspace permissions take priority over your TA permissions. For example, if you were added as a Standard user to a workspace, you would lose all your TA permissions on that Workspace. Your TA permissions would be reinstated when you are removed from the workspace
Being added to a workspace when you're a TA
A Tenant Administrator cannot add themselves as a workspace user. A TA can be invited to join a workspace by the Workspace Administrator, Manager or another TA.