Ufdjrw

I use such .htaccess rules to hardening wordpress and redirect to https:

# Block IPS for login
<Files "wp-login.php">
Require all denied
Require ip xxx.xx.xx.xx
Require ip xxx.xx.xx.xx
</Files>
#Protect wp-config
<Files "wp-config.php">
Require all denied
</Files>
# Prevent directory browsing
Options All -Indexes
# Protect htaccess
<FilesMatch "^.*.([Hh][Tt][Aa])">
Require all denied
</FilesMatch>
# Block Includes
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^wp-admin/includes/ - [F,L]
RewriteRule !^wp-includes/ - [S=3]
RewriteRule ^wp-includes/[^/]+.php$ - [F,L]
RewriteRule ^wp-includes/js/tinymce/langs/.+.php - [F,L]
RewriteRule ^wp-includes/theme-compat/ - [F,L]
</IfModule>
# Block xmlrpc.php
<Files "xmlrpc.php">
Require all denied
</Files>
# Security related. Block browser access to log files
<Files ~ ".log$">
Require all denied
</Files>
# Extra Security Headers
<IfModule mod_headers.c>
Header set X-XSS-Protection "1; mode=block"
Header set X-Frame-Options SAMEORIGIN
Header set X-Content-Type-Options nosniff
</IfModule>
RewriteEngine On
RewriteCond %HTTP_HOST ^domainname.lt$ [NC]
RewriteRule ^(.*)$ https://www.domainname.com/$1 [R=301,L]
RewriteCond %HTTPS !^on$
RewriteRule .* https://%SERVER_NAME%REQUEST_URI [R=301,L]

WordPress is installed on the main directory. Now we plan to install second wordpres blog in a subfolder (public_html/de). New installation will have its own .htaccess file. Do we need same hardening rules for this new wordpress installation or it is enough to have hardening rules in the main folder?

I think you need use
RewriteOptions inherit to use the parent's inheritance

Something else :
You do not use the comments # BEGIN WordPress and #END wordpress so that wordpress when regenerating your htacces does not overwrite your overload?