If your second subfolder installation is successful, you may likely run into an issue when WordPress writes its Permalink (within the subfolder). The issue apparently has to do with the subfolder installation and how it sees the site root path. WordPress will normally write a .htaccess file in the installation directory, and that folder is usually the website root folder. Here is the applied solution:
change to new WordPress .htaccess (within the demo subfolder)
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /demo/
RewriteRule ^index.php$ – [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /demo/index.php [L]
</IfModule>
# END WordPress