Since Agile Central is a SaaS tool, you can only use a single email as your username in a single workspace. To reuse that email address in a different workspace, you need to open a support case, IIRC. Note that this removes all access to the old workspace. If you need to keep both for some reason, you'll need to use different email addresses (even if both addresses wind up in the same inbox).
Actually, you can only use a single email as your username in a single subscription, but you could use it in multiple workspaces in the same subscription.
Here's more on User Name Conflict.
Yes, you're absolutely right. My bad!
Appreciate your help, though!