Separate registration and login for different roles

//Creating two separate registration for different roles is easy:
//http://wordpress.stackexchange.com/questions/15954/separate-registration-and-login-for-different-roles
//create a hidden filed for role
add_action('register_form','add_hidden_role_field');

function add_hidden_role_field(){
    if (isset($_GET['role'])){
        echo '<input id="user_email" type="hidden" tabindex="20" size="25" value="'.$_GET['role'].'" name="role"/>';
    }
}

add_action('user_register', 'register_role');

//save the the role
function update_role($user_id, $password="", $meta=array()) {
   if (isset($_POST['role'])){
       $userdata = array();
       $userdata['ID'] = $user_id;
       $userdata['role'] = $_POST['role'];

       //only allow if user role is my_role to avoid a few new admins to the site
       if (($userdata['role'] == "seller") or ($userdata['role'] == "buyer")){
          wp_update_user($userdata);
       }
   }
}