How to have different password for unpublished site from the site password

We have our published website and am now working on an as yet unpublished site, which I am doing for a friend, How can I create a different password for the unpublished site only that does not allow access to the published site?

Or, alternatively, can I migrate this unpublished site to a new site with a new email address and pass word?

Thank you for any help with this.

Any help with this would be appreciated! Thank you.