Where is the the site name and slogan set?
In your active theme's template.php, check template_preprocess_page() function and add following lines:

$site_name = variable_get('site_name');
$site_slogan = variable_get('site_slogan');
$variables['site_name_and_slogan'] = $site_name . ' ' . $site_slogan;


Assuming that you have used $variables as argument in template_preprocess_page() function.

Now $site_name_and_slogan can be used in page tpl files.

If you have not implemented template_preprocess_page() function, then replace template word with your theme name and add the function as mentioned below:

function THEMENAME_preprocess_page(&amp;$variables){
  $site_name = variable_get('site_name');
  $site_slogan = variable_get('site_slogan');
  $variables['site_name_and_slogan'] = $site_name . ' ' . $site_slogan;
}