Merging CodeIgniter and NoNonsense Forum

The file /application/.htaccess is designed to remove permissions that needed to be granted in the website public (root) folder (in your case C:\xampp\htdocs\ )

The .htaccess for no-nonsense needs to be in the root folder which is the same place CodeIgniter needs one if you want to rewrite url's to remove index.php. As you note, there can be only one file named .htaccess. All you need to do is copy commands from one into the other file.

I haven't examined it closely but there is a very real possibility that some of the rewrite rules and/or conditions will interfere with CodeIgniter. You might have to install no-nonsense in a subdomain so as to remove those conflicts.