Two licenses, two sites, why can't i register them under one account?

I have two separate sites with two different installations of newspaper, but at the moment of registration, it says that i can’t registered already under one account, why can’t I just register my two licenses under one account?

Go to your profile page in tagdiv.com, then go to “Edit account”, then “Product keys”, and add your new product key there. That way, when you register the key in your site, it will not ask for new user information, it will activate.