Yes, you can register more than one account with Clickatell using the same username (your email address). The account name will always be unique, e.g. you can register the account name AccountName1 and AccountName2 with the same username.
If you have multiple accounts linked to your email address, you will be prompted to choose an account when logging into the portal.
Note that you can add an unlimited number of users to your account without having to create more accounts. Find out more about inviting users to your account here.