There are two ways to start the Admin server. The first is by using the iTools "front-end" application to turn it on from the Admin menu. If it is enabled there, you only need to log in once in the browser to authenticate yourself as having admin access.
If you have not started the admin server from the iTools, the first password challenge is for permission to turn on the admin server.
Accessing the URL "http://yourhostname/itools_admin" can start up the admin server, as well as serve the admin home page for configuring iTools.
The first challenge is from Apache/iTools, and the second challenge is from the iTools/Admin Server itself. Web browsers will remember the username/password combination for that session, and you will not be asked to authenticate yourself until you quit and restart the browser.