Yes, you can register more than one account with Clickatell by using the same username (your email address). The account name will always be unique, e.g. you can register account name BLUE and account name RED with the same username.
If you have multiple account names linked to your email address, you will be prompted to choose which account to log into. However, you don't need more than one account if you want to add more users. Find out more about User management.