Login to the content management system with your username and password.
Select the website you wish to edit from the website title dropdown box at the top-right hand side of your editing area. All your websites listed under the one login will be listed in this area.
If you have not yet created your second site, simply log out, then go through the process of setting a free trial site using the details that you used to create your first site. The second site will then be automatically created under the same login as your original site and be managable under the same login as above.