Ok, adding a second (or third, or fourth) website to your account is easy. Just visit the dashboard, and click the "Launch" button for the app you want to use. A dropdown menu will appear, showing your current selection of websites, plus the option to add another one - check out the screenshot below:
Enter your desired new domain, and hit "Add Now". You'll then be briskly escorted through the same connection process you experienced when connecting your first website to your account.