Q. Can't login into another account's cPanel

A. To access another account's cPanel you need to log out (sometimes even close and reopen the browser) and enter the credentials for the second account. it will let you in with no issues. To avoid this is best to log in into your account manager from http://serverdomain.net/ 

with the credential we sent you. From My Account click on the View/Edit link for "Web hosting plans in your account" it will display the account you are hosting with us. On the right click on the magnifying glass to see the details of that particular account and then on the "Control Panel" link. It will take you directly to your destination without need to enter more credentials.