Protect the whole WP site

Hi All.

From a WP blog I’m redirecting the subscriber to another WP website that I would like to completely hide to anyoneelse than the subscriber.

To make it more clear. The user will click to a page in the first site that will require a user and password. Using a login redirection plugin I will move him to another website. My problem is that I would like to make the second website 100% hidden to any other visitor that may land on the home page. I’ve set my blog not to be indexed by search engines but I don’t think it is enough.

Any suggestion?

Thanks in advance.