Is it possible for an admin to create two different galleries under one gallery image?
If you are logged in as the admin there is no "Create Your Own Gallery Here" button when there is already a gallery there created by the admin.

The only way would be to trick the system and add additional by creating a new gallery that has the log in permissions set so that others can upload galleries and then log out, and use a mock user... such as another account that is set up with a generic "Administrator" name. I have seen other admins do this.